Okay; sorry it has taken so long to reply. I wanted to be sure I had the problem solved. This one was a tricky one because the problem was actually two-fold. i.e. two problems with similar symptoms.
1. I replaced the PTO Clutch. This was an issue for I have an older ~2003 Scag Tiger Cub. Reason: The new PTO Clutch comes with an anti-rotation arm; which if you have an older Scag doesn't fit out of the box. The PTO Clutch fits fine, but the anti-rotation arm had to be welded onto the back frame. Not fun, if you don't know how to weld or have a welder. Luckily I do.
Once replaced, the mower performed much better, but did quit after getting into thick grass or increasing the throttle.
2. Replaced fuel filter and used some dry gas. Seems the mower was really sensitive to the moisture I received from my local fueling station. I didn't think this was part of the problem initially because my cars were running just fine. Evidently, cars can withstand small amounts of moisture.
Problem(s) have been fixed. At least I have mowed twice with no problems.
Good luck.