Hello guys n gals, I recently purchased a 99 Firebird convertible. When I bought it seller had just put fuel pump in it because it was running rough. On fire up car ran smooth as silk, but after it heats up it starts running really rough. On inspection of the oil water is mixing in, I know this sounds like a blown head gasket but if you let it cool down it fires rite up and purrs like a kitten. Just seeing if anyone has encountered this and or confirm blown head gasket before I start repairs in that direction. Just seemed puzzling to me that it would run so smooth until thermostat opens.