bump for you cuz i'm stumped. i'm assuming you've checked all the obvious stupid stuff, like all those annoying little radiator hoses? there's a bunch of tiny ones in the back of the engine. you mentioned whenever you turn the heater on it bubbles...maybe the heater core isn't getting bled properly for whatever reason? what about a hairline crack somewhere in the radiator? my 95 ls, i changed a radiator hose and just topped off the coolant, didn't even bother to bleed it and i've had no overheating issues. i've even taken it on a few 4 hour drives (each direction)

good luck!