how can i stop my sintra over revving?

My vauxhall sintra started badley over heating 3 weeks ago. sent it to my garage. they replaced the head gasket and the usual bit's that get done when this is done. plus the cambelt. it has all been flushed through too including the radiator and pipes. it came back and is over revving at 2000 revs while idle. He had it back and had a look at the air intake valve etc and managed to slow it down to 1500 revs, but this is still annoying, does anybody have any other ideas please.???