I have a 2003 avalanche that had a 5.3 originally, I swapped to an 8.1 and reprogrammed the ECU using a Jet DST. It runs and drives well and fine my only issue is on initial start up the engine surges back and forth from 400-800 rpm for a few seconds than revs up to whatever the desired RPM happens to be. Any info on what parameters to change to get a smooth start up would be appreciated.