aprilia rs125 cooling problem?

right here goes............. rs125 52plate 122 engine
rebuilt engine, filled up cooling system and went to test my bike.
2 days later i pulled up to buy a cob for me dinna and i got clouded by stem.
i thought okkkkk! took tank off had a look and bleed it and filled it up.
rode it for ten min got off and the same thing happens again
found out the rad had a bleed bolt in it so i bleed the system again.
went for a ride and its doing it again.

whats happening is as if all the coolant is being pushed back in the the coolant tank,
and it forces its way out through the thred and goes on the engine.

when this happens i can get off the bike and leave it over night and the next day i still cant open the cap off untill i open a bleed point of if will gush out.
and the temp is running between 50-60 and if i stop at a red light is goes up to about 65 maybe more.
thought they ment to run about 65-75max

PLEASE HELP ITS DRIVING ME TOTAL NUTS