1999 Tahoe PCM Memory
 

Good evening,
So on my 1999 tahoe 5.7 i had to have my timing parameter set to make it run right. It was at 0 degrees at start up and as soon as you hit the pedal the timing jumps to -12 degrees. of course this makes the truck run like crap.so i had a guy that could access this and corrected it. Now as a few days pass it has made its way back to the -12 degrees. Is the PCM not holding memory or is there something else telling the PCM to go back to these parameters? Im totally lost guys. Any help is greatly appreciated.

1999 Chevy tahoe
2wd 5.7 liter