The case of the mysterious GFC interrupter

A few years ago, I had my kitchen renovated. This included replacing the old overhead light (regular bulbs) with halogen lights (the dangling kind). Everything worked well.
Recently, I've noticed something really bizarre. When I turn on the halogen lights, one of the GFC interrupters trips. The GFC outlet is a few feet to the left of the light switch. The bizarre part is that the GFC outlet is on a completely different circuit than the halogen lights. Yes, I've tested this with the breaker.
If I turn the light on, reset the GFC, and then turn the light off and on quickly (1-2 second delay), the circuit doesn't trip. If I wait longer, it trips. The transformer for the halogen obviously needs a second or so to "power up", so this is a clue.
There are two switches for this light, on either side of the kitchen. Both switches cause the GFC outlet to trip, so it's not the switch. Just to make sure, I checked the switch that's closest to the outlet, and the wiring looked OK (although for some reason, it is a 4-way switch, despite the fact that there are only two switches for the light)
The only thing I can think of is that the wires from both circuits run side by side in the ceiling, and the transformer is somehow generating a field that causes the GFC outlet to trip. Is this in the same category as aliens-have-infestated-my-house or is this at least a remotely plausible explanation?
I could test this theory by wiring a regular light instead of the halogen light, and see if it still trips. Before I climb onto the countertop, I'd like to make sure I'm going in the right direction.
Any advice greatly appreciated.
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My best guess is that the two circuits are sharing a common neutral. They shouldn't be, but it's possible that their neutrals may have gotten tied together inadvertently. BTW, if you've got a 4 way switch, you've got to have two 3 way switches. This could be a clue also. Was it an electrician that did this wiring?

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Thanks for the reply.
The work was done by a licensed electrician. He seemed to know what he was doing. Everything worked fine for ~18 months or so, then the problem appeared intermittently, and now the GFCI trips every time.
Oh, and there are indeed two 3-way switches in addition to the 4-way switch. It is behind a door that's always open, forgot it was even there. And yes, that switch also trips the GFCI.
Overall, the wiring in the house appears to be good. I've had very few problems in the 5+ years I've lived here (the house was built in '85). Apart from a problem with one outlet in the basement (which the previous owners "fixed" by replacing a 15-amp breaker with a 30-amp breaker.....not smart), everything has been fine.
Would a regular light (i.e. non-halogen) display the same symptoms if it is indeed a common neutral? How would I go about testing the common neutral idea?
Thanks!
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There really should be no connection between kitchen outlet (GFCI) circuits and any lighting circuit, except where they come into the panel. If one of these switches shares a box with any of the kitchen outlets, I'd look there to see if their neutrals got tied together

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Thanks RBM. Based on what you wrote, I suspect the problem is in the box where the 4-way switch resides. The GFCI outlets look distinctly newer than the other outlets, so my guess is they were added later to bring the kitchen up to code. There is no telling who did that job, perhaps tying the neutrals together was done as a 'quick' fix before they put the house on the market.
I'm out of daylight today, but I'll poke around next weekend. Thanks again for your help.
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snipped-for-privacy@gmail.com wrote:

So I took a look at both the light switch and the GFCI outlet.
The box that holds the GFCI outlet also has a dimmer switch (for a different set of lights). Talk about cramming wires into it! There are five separate lines feeding into the box, and quite frankly, I'm having a hard time making heads or tails of it. I posted the diagram at http://baspluim.googlepages.com/main (I named each line according to color/ position, which doesn't matter much in a flat picture of course). There appear to be five neutrals _tightly_ wound together using a single wire nut.
The wiring for light switch (that trips the GFCI when turned on) is pretty straightforward (http://baspluim.googlepages.com/kitchenwiring%232 ). I haven't yet figured out which line connects to which (there is another regular outlet between the GFCI outlet and the switch), but most likely it's the line with three wires (Black back)
Unless someone spots something obvious, I think I'll let an electrician handle this one. Just too many wires to keep track of, and not a lot of real estate.
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Let me throw in a wild guess. The GFI operates by passing both the hot line and the neutral through a ferrite core along with a sensing line. If the current is identical in both hot and neutral this little transformer has no output as the effective primary has no current (the two lines cancel out). If there is an imbalance because some current is leaking out a "sneak" path, the sense line acting as the secondary of the transformer now has an output and trips the GFI. Possibly, the rapid turn on of the high current halogens induces either a spike on the power or neutral lines that are sufficient to cause a trip or a radiated signal is being picked up by the sensing circuit. Incandescents may not present the same turn on characteristics.
Of course, I could be wrong.
Charlie
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