Its not a head gasket problem, and DONT FLUSH YOUR MOTOR EVER!!! not with bilstien or any 
like that, you will lose compression and shorten the life or your motor. and an engine flush would have nothing to do with this problem.
Who said to flush the motor?
And if you do it properly there is no problem in an engine flush, we are talking cooling here not sludge build up. You can and should flush the cooling system it won't hurt a thing and more than likely you may find this is the culprit. FYI I basically flush my motor just before every oil change with some transmission fluid. Funny I have almost 250,000 miles on my motor.
