Mower starts fine, but eventually dies.

I have a Toro Recycler 21" with the Kohler 149cc engine. The mower is about 4 years old, but very low hours. I got the mower second hand, and I know the original owner had let it sit for several years with bad gas. The mower was then sent to a professional shop to be cleaned out, and that's when I acquired it. It has been running fine since... for the first hour or so of use, before it eventually loses power and dies. My first thought was that the carb was still clogged, but a friend of mine took it apart and said it looked great. Any other suggestions of things to check?

I have tried loosening the gas cap when the issue occurs and it makes no difference. Gas is fresh, oil is new and proper level, and air filter looks OK (about time to be replaced but hesitant to waste more money on nonrunnning mower.)