I have a 2000 Sierra 1500 with the 5.3. Ext. cab, 4x4. The problem started when I got into stop and go traffic following my dad. I felt it missing when i would let off the throttle and when idleing. I drove around 6 miles before stopping. Got a few parts in a hardware store and left. In the next 2 miles it started getting worse. Then the Check engine light started flashing. Got it home, hooked it up to my brothers scanner. P0300 code was thrown twice. It was idleing rough, so i wanted to see which cylinder(s) wasn't hitting. Pulled the wires off the top of the coils one by one. only found 1 that didn't affect the idle. it's on the passengers side 1st one back from the front. not sure which number it is. Swapped sparkplugs from that cylinder to another didn't help, hooked to another sparkplug to the cylinder that wasn't hitting, and grounded it to the dipstick tube. started it and it had spark. Also I can smell unburnt fuel in the exhaust. Mods done to the truck are muffler delete, k&n intake, ported throttlebody, screen removed and ported maf. All done over 6000 miles ago. and took the egr out of the intake, plugged hole with 1-3/8 drain plug that seals perfectly. done about 2500 miles ago. Last thing that was changed was the fuel pressure regulator, my pressure was around 35 at an idle 39 under accel. new reg. it's 49 idle 56ish under accel. it was done around 60 miles before the problem started. I know that was really long, just wanted to give you as much information as i could. Any help is greatly appreciated.