Jumpy idle.

Lately, it's been really damn cold around here. It was about -12 tonight, when i checked the temps online.

My tach is jumping around between 2k and 1k when I stop sometimes. The water temp on the dash is the lowest it can be, like I just started the car, but infact, I had just ran the car at 70 MPH for about two hours. I am wondering if I have thermostat problems, or maybe the car thinks it's cold, then thinks it's warm and adjusts the idle accordingly. I doubt if any sensors have ever been replaced.

Ever had a similar problem? What causes this?

I'd say that it's probably a drivebelt tension problem or a thermostat that's stuck open. That's the only two things I can think of.
I'm thinking drivebelt because of the jumping in your tachometer. Good luck.