1993 Toyota Camry Expansion tank overflows when engine is n

I'm having engine coolant spillage issues. When the car is brought up to operating temperature and then shut off, the expansion tank expels coolant (quite a bit) in spurts. The spurts seem to be caused by the radiator cap oening/closing as pressure builds and drops with each expulsion. This occurs even after a short test drive to get up to operating temp. Note that there is NO overheating of the engine. It operates well within its desired temp range while driving. Here's some backgroudn info and things I've already tried:

I have replaced the thermostat. I have flushed and replaced the coolant. I have bled the system with the engine running and heater on to remove any air pockets. I have ensured that the expansion tank has the proper fluid level after the system is bled. I have replaced the radiator cap. I have replaced the thermostat housing cap. There is no coolant in the oil, nor is there any oil in the coolant. When this occurs, I let the engine cool down, then I refill and bleed the coolant. Next time it gets up to temp, it overflows again. Note it is not just blowing coolant into the expansion tank, but it's actually blowing fluid out on the ground from the expansion tank overflow. Ideas?