mercury thermostat problem

Hello all,
We've noticed that the blower in our house HVAC system has been running nonstop for a day or so. The thermostat was set to HEAT, fan set to AUTO, and the ambient room temp was comfortable and well above the thermostat setting. Logic told me the blower fan should not have been running.
I pulled the cover off the thermostat, and it has what appears to be a bi-metal spring coil with two mercury switches. Moving the mechanical thermostat lever up and down didn't seem to be doing much at all. I moved the lever way down to its bottom limit, around 50 degrees or so, and the blower just kept running for at least an hour until I intervened again. Then after some trial and error I discovered that if I pulled slightly outwards on the lever as I move it, then the coil/mercury switch assembly rotates with it.
After thinking about it for a minute, I realized that maybe that assembly is supposed to always rotate with the lever in order to adjust the desired temperature setting? After more trial and error, I think I've matched the temperature indicated by the lever location to the actual room temp (ie, when the ambient room temp drops below the indicated thermostat setting, the blower and heat pump turn on) by moving the lever with and then without turning the mercury assembly.
So now I think everything's working OK, except that I can't adjust the thermostat setting without removing the cover and pulling the lever outwards slightly to get it to "grab" the switches so they rotate with it. The heat pump comes on now when the temp drops below the thermostat setting, and it seems to maintain that temperature properly and shuts off when the ambient temp is reasonable. So it seems to be working except for not being able to adjust the temperature setting.
Does this all make sense and sound correct? Or am I out in left field? Any (helpful) suggestions are appreciated as to what my next step should be...
(btw, house is 14 years old, HVAC system is the original Rheem forced air heat pump, works like a champ knock on wood except for the thermostat problem)
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Get a digital thermostat. Robert Shaw and several other companies make digital thermostats. I have one in my house, with no "programs". It is just on, off, auto...you plug in the temp. I find it is much more accurate and the temps in the house stay much more constant. The plain one is about 40 bucks. Perry

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