Hello DaDave,
Sounds like you have tried everything already, so it would be a hardware escalation: 1-800-821-2797.
Intermittenly folds any size paper from tray 3 then the print job stops and control panel says "Wrong paper size , load tray 3 with letter".
Tray 1 and 2 work great.
Already replaced Feeder Assembly, p/n 059K26934
Tried swapping paper cassette 2 with 3
Nothing obstructing the paper path coming from tray 3