Hello everyone

I’ve got a Ford EL XR6 manual under load it performs great. As soon as I try to light cruise it continuously stumbles and misfires, then you apply some throttle and it performs great as soon as you back off it’s crap.

I had it dyno tuned and the guy said it’s definitely not in the tune or the fuel system, he said it’s definitely spark related.

Anyone come across this problem

He's not likely to blame the tune is he -scan for codes -was it playing up when he did the tune.
Check for play in distributor - check rotor and cap and verify carbon brush is in the cap.
How old are the plugs and what gap .
Have injectors been cleaned.
Have you checked for vacuum leaks.