'97 Golf Diesel TD

I've been having this problem in my car that after driving for a couple of hours in city the temperature drops from the normal 90 degrees to about 80 and stays going up and down arround that area but never over 90 or under 70, i took it to the mechanic and he told me that if it doesn't go over 90 it's fine but i dont find this normal. Any idea of what it might be?

Other than that i was hoping you guys could give me some heads up for any problems that this car might give the body has a lot of millage on it but it's in mint condition and the engine has 150,000 km on it. I already changed the brake cilynder, the block heater and now the wheel bearings are giving me problems.

Are all these problems gonna stop soon or are there many more to come?