Small Engines (Lawn Mowers, etc.): Peerless-Tecumseh transaxle problem, tecumseh transaxle, drive pulley

Question
A buddy of mine has a Murray riding lawn mower, automatic drive, 17.5 HP 42" cut, with a Peerless-Tecumseh transaxle.  Complaint initially was that after running for half an hour or so, the mower would not travel well, seemed to be slipping going uphill, for example.
 He changed the belt, but that didn't help.  Upon examination, the transaxle was very heavily covered wiht dirt, rendering the cooling fins useless.  The small fan under the transaxle drive pulley was slightly loose, but still functional.  I cleaned off all the dirt, tightened the fan, and now the unit will travel, but still appears to be slipping, and the transaxle is making an internal noise.  The belt shows no sign of slippage, no glazing, burn marks, etc. on the flanks of the v-belt.
 The transaxle has both Tecumseh markings (6733-P1 (R) cast on the case, but it also has a Peerless tag.  Appears to be a Tecumseh automatic transmission mated to a Peerless axle.  Mower model #425610X99B.  Online search revealed parts list, but that list had no transaxle break down.  List said to contact local distributors.
Is this a common problem?  Are these things filled with oil or rear end grease?  Nothing in the Murray owner's manual about it, of course.  I was surprised to see that little fan and all those fins--heat must be a real problem with the design.
 Thanks.
LtCol Rick Jones (USAF-retired) (B.S. in Mechanical Engineering, also, but I'm no mechanic)

Answer
First, find the numbers on the tag, that should have the model
number, and a blowup can be downloaded here:

http://search.cartserver.com/search/search.cgi?cartid=a-8671&category=TecumsehPD...

Murrays made during that time had a real thin/weak frame around
the area the tranaxles were mounted, and cracking was very common,
and when extra load was put on the unit, the cracked frame would allow the transaxle to flex forward and the belt would slip, not enough to show much sign on the belt, but would make it slip going up hills, etc..
  Also, a buildup of dirt would prevent the "bypass" lever from
closing fully, the bypass lever is the one that lets the mower be pushed when the mower is not running.
    Look at both of those areas, and see if you can read the model numbers and let me know.

  Thanks,

   Fish