Hello, I am the original owner of my k2500 HD. I'm an auto body and frame tech, so I'm somewhat knowledgable mechanically, and maintain my own vehicles. Truck is as follows 6.0 engine, 4L80 e trans, 4.10 rr axle. Mods are a K&N fipk2 installed at 25,000 mi, throttle body spacer installed at 25,000 mi, and a flowmaster muffler installed at 15,000 mi. It also has oversized tires. I bought the Hypertech power programmer to correct the speedo and also changed the shift too firmer and have had no drivability problems. since new I've only put in 89 oct gas. Truck has 90,000 mi on it now. Then in Nov 2006 at cold starts (40* and lower) truck would start easy and idle up but about 5 sec later the idle would drop to 200 to 300 RPM and engine would lobe. It never stalled and would always respond to the gas pedal. After it warmed up to temp, all is O.K. Eventually it threw a check engine light so using my Hypertech I retrieved the following P0300- misfire P0171 fuel trim lean bank 1 P0174 fuel trim lean bank 2. I changed the fuel filter (2nd one, first was at 40,000 mi) and cleaned the MAF sensor with maf sensor cleaner, then cleared the codes. Problem persists and lately the idle has been too high at cold start 1200 to 1300 RPM and hanging there until operating temp is reached.