Slow 600,
As Stewart just stated, it can not be running properly. There is NO way a Blue 426 can hang with a Big Red 500 or the Mean Green K5. You mentioned you checked the power valve operation (by hand?) and you are running the Gnarly pipe, Pull the pipe and LOOK in the port and confirm the barrel valves are opening all the way, and the center valve does not have play in it when open (stick your finger in the center of the port when you pull the rod, when it is open see how much play is in the valve). The Gnarly pipe is more for the off road / woods rider (good bottom & mid) you may consider another pipe (it will still yank the 426 with the Gnarly when running properly) check Stewarts post where he has already done all the work for us on Which pipe to use, the power valve overclosure bolt remedy, etc., etc., etc. Let us know what you find out.