try thermostat first vance ...... start simple then go to the hard .... good way to diagnose a problem instead of spending money for new sleeves and expensive crap when its a 5 dollar problem .,..... if it isn't the thermostat .... then do wut they said man .... but my z6 had this same problem i replaced the water pump, head,and radiator ... and it was still doing it .... replaced the thermostat .. and boom fixed .... trust me it was a headache .