after running the car and it not overheating at all, the overflow sometime bubbles. i know its not a head/gasket as i have no other symptoms of this. now, i was going to do the basic things first, burp the system, install new thermo, rad cap etc. but i do have a leaking heater core. well, its not leaking in the pass. area yet, but i do smell coolant when i slide the heater on. so if the heater core WAS leaking, is that a big enough source of air to cause the overflow tank to bubble?

might as well replace the heater core while u have the coolant drained out for the thermo install. refill /burp as needed.

the only time ive seen a over flow tank bubbleing is when my bird over heated. coolant might be to low also.

Even though you don't seem to have any head gasket symptoms, it would be worth using the tester kit with the liquid that turns from blue to yellow if you have exhaust gases in the coolant. NAPA sells the kit; other places might rent it (but I'm not sure about that).