Yeah, but he failed a compression test.If you still have the stock H pipe, a misfire will cause the same symptoms as a valve not seating. I just checked my 96 gt and that's what it was doing. I searched last night for other causes and quite a few people had a misfire and when that was fixed it was back to normal.
I don't know how a valve would "stick". The cam will push it open and the spring will push it shut. It mostly likely a burnt valve.
A collapsed lifter or broken spring would be fairly catastrophic and have more symptoms in the way of unmistakeable engine noises/damage.
Bad ring(s) can fail a dry compression test, but I don't think they cause exhaust reversion. Did you do a wet compression test? If you get more compression wet, it is generally rings.
Do you have adjustable rocker arms?
A rocker arm adjusted too tight can cause issues.