I noticed that your question has been open for a while and, although I am not a repair person, didn't want you to have to wait a terribly long time for an answer.
So- I checked with my husband for you who is a part time landscaper (full time electrician).
He said that it sounds like the belt started to slip when you hit the heavy grass because it put such a burden on the blades. IT reengaged and started back up but then slipped again when faced with the same burden. This will cause the belt to stretch slightly and it's just enough to cause the belt to be ineffective even though it is still on and looks like it should function.
He said getting a new belt should solve your problem. You also said that you might want to attach the thick/high spots with a weedeater or push mower before you use the craftsman.
I hope this helps.