I have just three months ago bought a 1981 300TD with 126k miles on it. I am new at this and am looking for some advice.

I have an intermittent problem with overheating with my wagon. It is absolutely weird in that in town and highway driving the temp will go slightly above 100 degrees, then drop down to about 90 degrees, then work its way up again to 100. On the flip side of it I might go two or three days of the temp never going above the 80 degree mark. I check my coolant, and all fluids on a regular basis. I replaced the thermostat a month ago when this started still with no difference. Two months ago I was getting a whinnying sound when I put the Air Conditioning on, and I am wondering if this is related to the problem.