HELP! Tenant calling with Surging Hot Water in the Shower problem?

Help! - My tenant is calling me with a real stickler of a plumbing problem and need help diagnosing and fixing / solving this one.
DAY1-I get a call that the Hot/Cold water surges in the 2nd floor bathroom (duplex/upper flat) shower. The Tenant downstairs is out of state so that person can't be flushing the toliet or anything like that
(rules out the downstairs plumbing activity).
I inspect the case: I go to the shower in question, turn on the water to a steady warm condition, and then go to the bathroom sink and turn on the cold, feel the bath water (remains the same). I go to the kitchen and turn on the cold (bath remains the same), turn on the hot, (bath water remains the same)???
I go to the basement to check things out with the water heater. The water heater is leaking bad from the bottom and is SHOT. I replace the water heater with a brand new 50gal/water heater.
3-4weeks pass, I think - replacing the broke water heater must of fixed
that problem.
Well I get a call from the 2nd Floor Tenant again today - " The water is surging hot/cold (I don't get the exact behavior but get the gist of
the problem) still inside the shower" I believe the hot water gets too hot and burns the showeree or it gets too cold one or the other.
I lived in that apartment for 9mon to 1 year and never experienced this
"surging shower water problem." Can't be the person down stairs either.
The only thing I can think of is that this scenario is happening:
Guy takes shower, turns on hot, after 5 min the H20 Heater demands cold water to fill the tank, this cold water depletes this guy's "cold" water in the shower and thereby causes a surge in water temperature?" Do you think that could happen or has that ever happend to anyone?
I need some help diagnosing and fixing this thing for good. Any advice and insights much appreciated.
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On 30 Sep 2006 21:22:39 -0700, snipped-for-privacy@hotmail.com wrote:

This problem is most likely caused by the packing/washers in the faucet to the shower head failing and letting cold water over to the hot side.
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Your incoming cold supply is virtually unlimited compared to the shower. I don't think your diagnosis is correct. Try the same test on a sink, and if that same problem doesn't occur, it's the shower faucet... good luck
snipped-for-privacy@hotmail.com wrote:

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Be helpful to know which, don't you think?
Is it just the shower, or does the tub spout do the same thing? Which thing? Stick your arm in for 6 minutes and find out.
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Goedjn wrote:

I'm going to try to get over there this weekend and do that exact check (shower vs tub facet) and ask as well. I'm going on the center diffuser rubber back washer is cracked or broke. I'm going to replace both the hot and cold black rubber washer and do that "drill the washer seat inside the stem" operation too. After inspection and maintenance of this part of the plumbing, all three stems (I think there is), I hope this one gets closed and will update on whats found and after effects.
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The water heater is demanding cold water to replenish what is going out the shower head the second he turns on the hot, so this can't be it.
The only thing that would happen after 5 minutes is that the temp in the tank drops below a threshold and the thermostat calls for heat (elements turn on, gas burner lights, etc.) which would have no immediate effect on the temp of the water.
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Many shower faucets have a pressure balancing unit or similar that will compensate for a difference in water pressure (flushing toilet)that moves inside the valve. If it is failing it may not move the way it should, causing it to not move at all to the change in pressure, or may stick and overcompensate, stick and overcompensate in the other direction.
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