gas furnace comes on erratically

Hiya folks -
I have an old gas furnace, original to my 30-year-old house. It has a standing pilot and is in good shape, and I have a digital programmable thermostat (make unknown). The problem is that the thing won't come on reliably, and there doesn't seem to be any pattern to it.
I have the thermostat programmed to raise the temperature to 70 degrees at 5:30 every morning. The stat flashes the word "Heat" on its display when it's trying. Some mornings it works, and others it doesn't. Even when the furnace doesn't come on, the thermostat is flashing "Heat" as if it's working. The pilot light is always lit. I've found that if I take the thermostat off the wall to expose the two wires coming up from the furnace and I short across those two wires with another wire and hold it there for a bit, then usually the furnace will come on after a while. If then I plug the business part of the thermostat back in, it will work properly. Then next time it's supposed to come on, maybe it will and maybe it won't.
How is this supposed to work? What should the voltage be across the two wires that come up from the furnace? Does the thermostat send voltage down to the furnace when it's signalling it to come on, or does it just make a connection between these two wires for the duration of the heat program? If I measure the voltage across these two wires, I get something very low, like 0.2 volts DC. Have I got a faulty thermostat or is there something wrong down at the furnace?
Thanks a million -
Edge
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You should have 24 volts ac. (measure again with the meter in the ac mode)
If the voltage is still low, check the wiring for a bad connection. (this can cause an intermittent problem)

No problem, just send a half a million, please!!! :-) (want the address to send it??)
Better yet, I'll pick it up personally. :-) (wouldn't want it to get lost in the mail) :-)

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Thanks for the tip! It was a loose wire, where the thermostat connects to the gas valve.
Hmmm. Half a million? Maybe I can talk you down?
Edge

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