Temp Guage Jumps

[BRIAN123]

My temp gauge needle flickers. Dead cold it reads 40, most of driving time it is around 80, and then it jumps between 80, 90 & 100. Sometimes it is pinned at 120. It never overheats or has any fluid leak. Coolant level is steady. My driving style or use of the heater has no affect one way or the other.
Where should I start?
What danger lies ahead if I do not fix it now?
Pennsylvania climate still in the 30’s for another week. It is kind of hard to work on in the garage.
My baby is a 1990 Sl 500 with 106,000 miles