zx2r cooling issues

hey I recently went for a run up mt nebo... half way up the temp guage ACTUALLY began to work.. and on the way down after being stuck in traffic the temp just seemed to hang above, or around half way.
I got the thing home thinking it was just from sitting in the traffic and wen I went down the next morning to check the coolant, I took the cap off and there was heaps of pressure in the radiator (it was mostly air). I topped it up, bled it, did a run to work last night in the icy weather (no rise in temp at all untill being stuck behind a freakin 3 lane wide catamaran on the back of a truck!) same thing this morning. the radiator was low and there was air when I bled it.
It doesnt leak anywhere. i suspect it could be the radiator cap letting the fluid into the overflow bottle (did boil once before) but not letting it drain back into the radiator??
any ideas I dont want to kill it ! but its my only transport
I got the thing home thinking it was just from sitting in the traffic and wen I went down the next morning to check the coolant, I took the cap off and there was heaps of pressure in the radiator (it was mostly air). I topped it up, bled it, did a run to work last night in the icy weather (no rise in temp at all untill being stuck behind a freakin 3 lane wide catamaran on the back of a truck!) same thing this morning. the radiator was low and there was air when I bled it.
It doesnt leak anywhere. i suspect it could be the radiator cap letting the fluid into the overflow bottle (did boil once before) but not letting it drain back into the radiator??
any ideas I dont want to kill it ! but its my only transport