Diesel Bombers - Please help -can bus?

2011 2500, deleted, headstuds, fass fuel pump, EFI Live, road ripper 3000 trans, new TIPM, new aftermarket alternator, batteries check out ok with clean connections. Had LED lights but took them all out as recommended trying to solve my issue.
The issue is a very brief stumble under acceleration particularly noticeable when loaded. A trans shop put a computer on it for a test drive & saw voltage drops to the TPS (assuming accelerator position sensor on this truck). Put a new pedal assembly in it - no change. Voltage drops to 12.2 - 12.7 then back up. Even does it at high idle on cruise control when parked. Please help, I dont trust the shops around here, they just want to throw parts at it...