Galant GSR 91 electrical Problems

Hi,

my little galant is suffering during this harsh winter and needs your help....
I've got a few questions regarding the iddle of this type of car.
When I crank up the car, the iddle is very high (like 2500 rpm) and then gradually goes down to 1500 after a while where it kindda "jumps" all the time.
I went to mechanic and was told that there is nothing you can do without spending a lot of dough on the ECU and other parts.
However, it seems this problem is only appearing in the winter and is temperature related.

Could this be the coolant temperature sensor that is busted and is causing this not very funny behavior?

Or another sensor?

How do I test them?

Another thing is the heating, it's working when the car is iddle, but when on the highway, it seems no to work and the car temperature never increases (unless left at iddle for 10 minutes), can this be related

Thanks for your help

Pascal from Michigan