Small Engines (Lawn Mowers, etc.): Yardman/Peerless? transmission, interference fit, drive gears

Question
I have a Yardman 604H 46" triple blade lawn tractor.  This has a single speed transmission fitted with a variable diameter pulley.  When going a slope every few yards it lurches forward quickly for about 6" before resuming normal speed.  I am wondering if the main drive shaft or the differential ring gear is mission a tooth or is there some sort of interference fit on a gear that is slipping.  It runs smoothly up the same slope.

The transmission has a part number (MTD's?) of 618-04034 which I believe has been replaced by MTD-753-05851.  The casing itself seems to have 718-04018.  

Any ideas as to the cause of the problem and is this a Peerless transmission?  If so what is the Peerless model number?

I can add further that the transmission has been getting progressively noisier from about 150 hrs on.  The mower is now up to about 240 hrs.  I don't want the mower to suddenly freewheel down a slope.

Any assistance would be gratefully received.

Malcolm

Answer
I would guess that the clutch in the transmission is getting worn and is slipping occasionally. It is a three pronged piece of cast steel that slides on the shaft that the forward and reverse drive gears are on. This transmission was made by mtd, and some of these parts are not available any longer, so you may end up having to purchase a whole new one.