I've had a intermittent or random misfire and extreme loss of power recently for the past couple of months. One time it through a Check Engine Light and it was a P1131 then the light went off on it's own. It has never come back even when it is losing so much power and you can feel it misfiring. I'm not sure why the Check Engine Light isn't coming on when you can actually feel the engine misfiring. I was told by a few mobile mechanics that looking at that code and doing a scan with their scan tool it was a fuel related problem either the fuel pump or fuel injectors. I've checked the fuel pressure everytime it messes up and it always comes out to 65-68PSI which is within the specs. I can't catch it when it happens because it goes away after you accelerate. So I thought maybe it's actually ignition problem so I swapped out both coils under warranty. Then it still did it. I tried bottles of ISO-HEET thinking it had water in the gas. I even replaced the EGR valve. Then finally I swapped out all 4 injectors this weekend. It has a lot more power but after getting off the highway and coming to a stop it was idling and you could hear it misfiring and skipping. So I held the brake while at the light and revved the engine a few times and you could hear it struggle then when the light turned green it was fine. I did a tune-up at 300,000 and it's got about 18,000. What else could cause a misfire without it being picked up by the check engine? The engine has about 150,000 it's been swapped out once. Timing belt was done about 10,000 miles ago.