HELP! I posted once before and have done everything I can think of nobody can seem to help me with problem. Trouble codes 1) P0421 - Warm up catalyst below threshold(bank 1) 2) P0300 - random/multiple cylinder misfire detected. Only happens at 70MPH. Replaced all vacuum lines . Replaced EGR Valve. Replaced plugs/wires. All new. Replaced fuel injectors (ALL 4). Had intake manifold cleaned out. Replaced Pre-cat(the one that bolts directly to the exhaust manifold. Replaced BOTH O2 sensors. Had exhaust system checked for leaks. Have taken it to multiple mechanics. No mechanic can find any other problems. Have spent endless amount of money to try and fix problem. Ready to blow the damn thing up. HHHHEEEELLLLPPPP!!!!!! I'm frigin losing my mind. After i replaced the pre-cat it ran fine for about 3 days then started to act up again. At 70MPH it kicks down no power with a few kicks of the gas pedal it kicks back in. As long as I keep it over 70MPH or under 65MPH its fine. Any help would be appreciated..