Mechanic says it’s not the head gasket.

[mizuboi]

by burping the system we mean raising the front end as far as it will go and run the engine for about 20 minutes and then fill the coolant as necessary that should help otherwise you might want to check the connectors onthe thermostat. check the dual vaulve on it too you know the blue one with two ports comming out of it...well thats all i can think of off the top of my head

and yes a bad bent old radiator can cause this also clogs in the places where the coolant goes

my girlfriends na did that for a while it lost coolant in teh system somewhere and then an hour or two later it threw it all up out the overflow tank so burping solved that well hope everything works out for ya