coolant issues, or rebuild required? :S

[hcDave]

I have a 1986 RX7 (non turbo) and it's acting strangely... It used to be that when I was on the highway it would overflow the coolant and give me the low coolant warning and I would have to stop, let it cool down, and refill it. This only happened once in a while and never while driving around town. Then it started to occur more frequently on the highway (every 50km of constant travel) and finally, now it's occurring around town

I had the radiator rodded and have replaced the rad cap.

I have done some research, and think it is either a bubble in the coolant system (there is supposed to be a kit which resolves this - if anyone knows of a link to this, please let me know) or a seal has failed (is there a way to test for this without pulling the engine apart?)

I have run the engine with the rad cap off and added coolant until there are no more bubbles present - this seems to help, but does not resolve the issue. I can also idle the vehicle up to operating temperature and it does not overflow (within 15 minutes of starting the vehicle anyway)

Any comments and suggestions are most appreciated!
Thanks in advance.

Dave.