ok guys i'll be the first to admit i'm not an expert at tuning gas engines, but i don't do anything too fancy, just advance timing conservatively, lower PE enable threshold, and firm up shifts. Have had good results so far. Now i tuned a 2006 silverado with 5.3L and after tuning sounds good at initial startup but within seconds looses power, throttle response and won't run over 2000RPM. Also getting EVAP code. tried lots of things with no results, but tuning back to stock and runs like normal.

Any ideas?