Geeez ya got yourself a tough 1 there. The head gasket and/or head could be leaking coolant into the cylinder itself causing your misfires til they clean themselves off especially with the miles involved. The loss of coolant could be minimal and still cause problems. The overheating is obviously something the other owner may have run into since he changed all these parts. I would'nt let a dealer change these things since they get top buck. You need to find yourself a good local guy as I have found. Good used intakes and heads can be found. The newer aftermarket head gaskets are soooo much better its rediculous . Its the labor on these things that kills ya once you find the problem. I'm sure others will post for U other options. Good luck welcome to GA fantasy world.