Random overheat

Hey guys, I need some feedback- the shop where I took my car is stumped.

Short version:

My 97 2.2 will overheat randomly when I come to a stop, but will return to normal once I get up to speed. The heater does not work at this time but will work otherwise. Everything else works.

Long version:

About a month ago I got a bubble in my radiator (overflow tank got too low). I refilled and burped. Worked fine but notice I had small leak in radiator. Got the radiator fixed.

Now the car will overheat but seems to happen only when I get off of the highway on my way to work in the morning (after about 20 min of driving). I brought the car to the shop. The car did not overheat when they had it, although I brought it in hot yesterday. They have checked:

Compression

Thermostat

fluid levels

Fans turning on

Heater core (based on something I found on another forum).

Water pump

Everything is fine. They do find if they let it run with the radiator cap open that occasionally bubbles will show up. Sometimes fairly big, sometimes tiny.

Current guess is hairline crack in head or head gasket but the issue there is that the bubbles are not consistent and the car passes all other tests. Any thoughts?

In my experience if they system has air trapped in the crossover it will get hot at a stop after being loaded for a while. As to where you are losing coolant to allow that bubble. Hmm. Get one of the chemical testers to check for exhaust gases in the coolant. A micro leak in a head gasket could do it without other symptoms. You could also get some dye in your coolant to do a blacklight check around the outside of the engine.

Mine is currently off the road with what I believe to be a tiny leak at the water pump shaft seal. I haven't bothered to pull the covers to check as I can just drive the other car, but it would sometimes leave a silver dollar of coolant on the ground right after driving and have no symptoms the rest of the time besides the hot at lights. Finally I spotted coolant coming from inside the timing cover at the bottom.

I wouldn't really expect a crack in aluminum without it failing completely.