About 3 weeks ago my car overheated on the freeway, pulled over real quick and let it cool added water and went on my way. After it overheated it was running rough, So i changed plugs,wires, coil still no change. Called a buddy who works for Ford he threw it on the computer and i found out it has 2 dead cylinders, Cyl 5 & 6. My question is: What would cause the 2 dead cylinders? He said possibly burnt valves, or head gasket. The car runs fine over 1k rpms but getting really bad gas mileage. How much will this cost to fix myself? and how hard is it to do?