I'm new to the forum and really hope I'm posting properly. I have a 2003 Impala with 3.4 engine. When I purchased the car, it ran fine for a day and a half then starting missing horribly at idle, cutting out as if I turned the key off while driving then cutting back in hard, and will often rev itself up. When it's acting up, if I bump the throttle to keep it from choking out, it Will often run itself up to over 2,000 RPM and stay there for a minute. The codes that were thrown when I started to diagnose it were P1635 (5vref circuit 1) , P0300 (random misfire) and Cooling fan 1 control circuit ( I replaced a bad fuse and corrected that one.) After more tinkering and testing, the P1639 (5vRef circuit 2) code accompanied the P1635. Alldata says that if there is a short to voltage at fuel level sensor or intake air temperature sensor, the P1639 and P1635 will show together. I cant find a short to voltage at either place but I'm not positive that I've tested it 100% accurately. Alldata calls for 4.8v-5.2v at the sensors on the 5vref circuit. I have 5.3v on all of them except the fuel tank pressure sensor which shows 5.2v. I have a cheap meter so I'm not sure if I should call the 5.3v out of spec. The missing doesn't seem to be consistent as it will sometimes run for a solid half hour before acting up. It's been in my garage for a week so it doesn't seem to be related to weather. I noticed along the line that I did have a plug wire firing off from the boot against a bracket. I taped up the boot and later observed it firing off again seemingly from the plug itself or the very bottom of the boot. I taped up the porcelain on the plug and haven't observed it anymore. It has crossed my mind that the plug firing off could cause a "hot ground" situation since it was firing against the engine but I don't know how likely it is to cause the issues at hand. Ive also noticed that the fuel gauge jumps around quite a bit while the car is sitting still. It's not erratic, but it fluctuates often between half a tank and 3/4 Tank. I'm stumped and cant afford to throw a bunch of parts at it that it may not need. I have replaced the IAC for peace of mind. I have sprayed Ether around intake gasket areas and all vacuum hoses I could get to and didn't seem to make any change. I've thought about throwing a fuel pump assembly and plugs/wires at it but I don't know how likely it is that it will correct anything. Any help appreciated.