Small Engines (Lawn Mowers, etc.): Craftsman self propelled mower problem, craftsman model, worm gear

Question
I have a Craftsman model #917.378840 that is self propelled.  At times, it appears as though the transmission is slipping out of gear.  It will stop the foward movement and act as though it is trying to catch.  I have replaced both front tires and the pinion gear on both sides (hoping they were just worn).  No luck!  It still slips.  Any advice?

Answer
 Hello Don:

 Is the Belt Tight? It Sounds like the Worm Gear May be Stripped or the Thru Shaft Key in the Transmission May be Sheared. Hope this Helps. I am here if you require more Assistance. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John