This doesn't sound like the OP's problem, but I just thought I'd throw
this out as a FYI. About 12 years or so ago, I ran into a round
Honeywell T87 that I had installed the year before, and something had
happened to the mercury. Instead of it being a shiny silver, it looked
like molten lead, and acted about the same. Normally,as soon as the bulb
gets the least bit off level either way, the mercury will roll in the
lower direction and turn the unit off or on. On this, it wouldn't move
until the bulb was slanted all the way, and it moved like a glob instead
of a fluid. For all the temperature control it did, it might as well
have been a toggle switch. I replaced it under warranty. But the really
strange thing was, over a relatively short time, I ran into two others,
both also T87's that were exactly the same way, and haven't seen any
more since. They always say things like that come in threes. FWIW, the
first was on a gas wall furnace, one of the others was on a gas central,
though the call was for the a/c, and the other an electric central, and
I don't remember if it was an a/c or heat call. Larry