What could be the problem?

I have a 1997 K1500 with a new 5.7L Vortec 5 speed transmission that had 0 miles on both when I installed them a couple months ago. I bought the truck as a roller and never heard the old motor run so I am not sure if these problems were present before. I am having multiple problems with this truck and trying to get it running like it should. It has a surge at idle when I first start the truck for about 10 seconds it fluctuates between 500-1,000 rpm and then levels out at 600-700. It is fine until I start driving it then the throttle will rev back up to 1,000 like the cruise is stuck or the throttle is stuck open. I have replaced the IAC, and TPS. I installed an aftermarket gauge column with the oil pressure and another for the coolant temperature. When I put the new motor in I put new plugs, wires, cap, and rotor. I am running it with the cats still and some aftermarket short tube headers and a 2 1/2 exhaust with flowmasters. The other issue I have is intermittent and feels like it is loosing fuel pressure but I tested it with a gauge and it reads 52PSI but it also was not acting up at the time. I could be driving down the road and it will start bucking like crazy but very intermittantly. I would think if it were the cap and rotor it would act like that all the time and the same goes with the plugs and wires. I consider myself to be very mechanically savvy but this one has me stupmed. I have even gone to a frind of mine and we hooked a brick up to it and he does not se any codes that would help him pinpoint what the problem is. Thank You, Jeff