may be an air lock in the system? let engine get up to temperature then SLOWLY AND CAREFULLY unscrew the lid on coolant bottle to release the air being careful not to unscrew too much for obvious reasons. if there is air in there, the coolant level will drop so make sure you refill up to the level, use warm water to be safe if theres a big drop. its worth a shot anyway, it cured an old astra i had!
