Craftsman T1000 belt-drive mower clutch pedal hangs, causes belt slip

We're having a problem with our Craftsman T1000 riding lawn mower. (Actually a rebranded MTD 247.203704.) It's a belt drive model with Zongshen engine and dashboard plastic lever to select transmission speed.
 
What happens is when starting out in forward or reverse the clutch/brake pedal does not move back all the way and the mower moves at low speed, probably due to belt slippage with the "clutch" only partially engaged.  Then after a little while the pedal kicks back the rest of the way with a clunk heard under the mower and full drive is engaged. This started out as a minor annoyance but is getting worse as with time as the mower is used. (Today it would not even move uphill when this happens.)
 
So it appears that the clutch pedal linkage is hanging up somewhere. I've looked under the mower and don't see anything obvious wrong but I'm not familiar with the mechanism and of course it's a jumble of control rods, springs, and levers. Anyone familiar with these have an idea of what's going on with this thing and how to fix it?