2005 Chevy Avalanche

My avalanche is loosing power completly it will not go past 2000 rpm, and does not shift no codes come out on dtcs I took it to a transmission shop, the tech doesnt think its transmission related, I have a hypertech chip intalled in it but before I took it in I set it back to stock settings, the problem comes and goes and then the truck runs perfect was wondering if its a silenoid, sensor, or computer related some times it does it right away or it could take several miles.