r/u_btrebesch

I recently bought this 98 GMC with the vortec 454, it runs and drives great other than when it’s going through its “cold open loop”, it runs extremely rich then suddenly it snaps out of it and runs like a top till I turn it off again. Previous owner did a very low budget intake and exhaust which I replaced/fixed, but not back to stock. I believe the issue is computer related but can’t find a shop that will tune this truck. Looking to see if anyone else has dealt with this issue and for any advice. Thanks