I thought the Radiator shop fixed the missing antifreeze! If Not, then as Croatan Kid, says "intake gaskets". Had that happen to my 350, which are Famous for bad gaskets! I also started getting a misfire on # 5, which is where the gasket was bad.
It can also cause engine to run erratic. Another misfire problem I had, under high RPMs, & towing, was due to my in-tank fuel pump, dying. Engine was simply running out of fuel. A Good clue to this, om GMs, is the fuel gauge will be acting faulty!
