Need new heater Thermostat -- or Fix?

It's fall, I'm trying to get a bunch of things done before winter sets in.
My OLD Lennox Electric Forced air whole house heater (was installed in 1970) has the original Lennox Thermostat.
It's the round dial-type with a slider on top of the dial that you set to desired temperature. Mercury bulb inside. It worked well until the end of last winter, when I noticed that if I set it at 68 degrees, it might go to 72 degrees or so until it shut down the heater.
Wifey has turned the heater back on and I am back with the beast not shutting down at 68 degrees (Oh, it will occassionally, I think).
Anyway, is there a way to fix this....or do I just need to buy a new thermostat. If I buy a new one, any suggestions? I do not want one of the programmable "bells and whistles," model.
Just another basic would be fine. Meanwhile, wife is very happy with the heater shutting off in the mid-70s range...me, not so much.
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tim birr wrote:

make sure that whatever holds the mercury switch to the spring is not letting it slide around. Dito for whatever holds the spring to the case. Clean the gunk out of the spring (bi-metalic temp sensor). As long as you don't break the glass, there's hardly anything to go wring.
I got a programmable one with the new furnace. I started out setting it back to 65 at night, but soon found that I was leaving it at 65 all the time and manually poking it down lower if I'm feeling too hot. If your house is "leaky" and you're not home (predictably) most of the time, timed setback might be useful.
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On 10/31/2011 2:55 PM, tim birr wrote: ...

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You can try the cleaning mentioned, but it's not likely going to help.
If the mercury isn't nice and shiny any longer, it becomes more "sticky" and it takes more to get it to flip, in effect increasing the hysteresis. Happened to the old classic Honeywell here a year or so ago; I put up with it until changed out the old system this year.
I looked some locally, the old-style are essentially a thing of the past and while still possible, seemed that they were more expensive than the inexpensive digitals. Contractor used a plain-vanilla White-Rodgers (sp? "d" or no-d?) on these systems; seem fine. No idea what they are cost-wise...
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Seems like a cobweb would produce the flaky results the OP is seeing,
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