Small Engines (Lawn Mowers, etc.): Craftsman/AYP 917.372870 mower rear drive sticks engaged, drive pulleys, clutch lever

Question
The rear drive system on my mower engages/disengages properly at the start of mowing the lawn, but after a few minutes of mowing, the drive tends to stay engaged regardless of the clutch lever position. If I jar the mower the drive will disengage. I've just replaced the drive control cable assembly again (they break every few years), and attempted the adjustment procedure several times. Is the problem with the drive unit? Can it be repaired? The $137 replacement drive unit cost would mean the mower is not cost effective for me to repair.

Answer
Since the cable is new the next part I would examine or replace the the belt.  The belt may be wore or the drive pulleys may be causing the drive to stick.  Looking at the parts diagram the transaxle is a 3 speed unit.

I believe the transaxle pivots back when you engage the drive system.  Pivoting back causes tension on the belt engaging the drive.  Is the transaxle pivoting back and fouth or is something binding causing it not to pivot correctly?  Also, is the belt tension srping weak causing the problem.
Let me know what you find.
Eric