Hello again. I have been unable to fix this problem. I fear I was unclear in my previous description.
The easiest way to understand the problem is to watch the following video of a young man attempting to use the mower. Just follow the imgur link below. In case you are unable to watch a video, I will attempt to describe the problem in words:
1) "Neutral" throttle setting puts the mower in reverse after a pause and a jolt.
2) Mid-throttle position lurches forward, pauses, and then lurches forwards again, then pauses....
3) Full throttle runs smoothly after a hard jolt
4) "Reverse" throttle setting runs smoothly in reverse after a hard jolt.
Two posters above suggested that I inspect the drive belt and pulleys for wear. The belt appears to be fairly new, and the owner tells me he changed it within the past couple years. The pulleys look ok to me. I have taken photos: