97 Civic HX with 256K

I just had the timing belt/acc belt/water pump/coolant change service done on my car.

Before the temp gauge would never move, it would stay in the middle.

Now it will randomly climb upwards to about 2/3rds and then come back down after several minutes to the middle.

Is there air in the radiator? I don't think it would be a head gasket leak (I sure hope not) b/c 1) it started after the service above and 2) it never climbs all the way up and always returns back down to normal.

What gives - any ideas - it still makes me nervous with all that mileage on my car...thanks