tbi 350 coolant boiling after shut down

[heff]

hopefully someone here can help with this, since i've read a million different things on a million different sites.
anyway, i have a 93 cadillac with a tbi 350. runs really good. no unusual noises, knocks, pings etc.
when i turn the car off, the expansion tank fills up with bubbling/boiling coolant. sometimes it will fill up to the top of the tank, sometimes it won't. ive replaced the thermostat(which i tested in boiling water with a thermometer to make sure it opened at 195) and i replaced the radiator cap. so far i've read everything from head gaskets to cracked intakes to bad radiators.
this car doesn't smoke any color of smoke, no hard starts, no apparent overheating problems when running or sitting in traffic with the a/c on. upper radiator hose gets hot, but not so hot you can't keep your hand on it for a minute or more. lower radiator hoses are cool to the touch tho, which i think is kinda odd. this car does have an oil cooler and a transmission cooler which is all built in with the radiator.
could the combination of hot oil and trans fluid sitting in the tanks cause it to boil out some liquid into the expansion tank, or could this be another problem? i have no problem replacing the head gaskets, i'd just rather know for sure before i pull the heads off an engine ya know?
my pontiac did the same thing when i rebuilt the engine the first time around, but when i tore it down(main bearings) there were know signs of a bad head gasket.
also, i did drain and flush the radiator. there wasn't/isn't and oil in the coolant. i changed the oil last week and there was no coolant in the oil. i did noticed the coolant flow in the radiator looked kinda weak, but that's in comparison with my pontiac and it's cast iron impeller.
so, to make this long story short, anyone got any good ideas?
thanks

[engineer]

any chance you have too much fluid in the system? wrong antifreeze mix? wrong pressure cap? electric fan does not run after engine is shut off?

[heff]

i thought maybe it was too much fluid in the system, but once i shut the car off, and it bubbles, then cools down, the level in the radiator is pretty low. but, i could misunderstand how this system works. as for the cap, it's the correct one for the car. and no electric fan.
thanks

[sleepy]

Your radiator is low but the over flow is full? Try this. fill the rad to the top put the cap on and fill the overflow to the COLD mark. drive the car until fully heated. let it cool down a couple of hours then check the rad. It should still be full to the top. the overflow should be at or slightly above the COLD mark.

[goquick]

Have you had the entire system flushed out? Also it is not uncommon for the heater control valve to hang up almost closed. It can cause a problem that might otherwise be considered a thermostat problem.
One other thing is how is the clutch fan? and is it a) working at low speed to engage, B) front of your raditor/a/c condenser clogged with bugs, road grime (especially after winter and road saltings)?