By far the most common cause of those symptoms would be a slightly slipping drive belt. Often that is misdiagnosed as a transaxle problem since it may not be evident that the belt is slipping unless it is under a load.
The next potential problem would be overheating. That might still be related to a slightly slipping belt that is causing the transaxle to work much harder than it should have to or it might be related to debris build up on the transaxle or a plugged or blocked or damaged cooling fan.
The oil might still look okay but is likely no longer good. If the transaxle has not been slipping due to an oil issue, that slipping will generate a lot of extra load on the drive train and that will eventually overheat and degrade the oil.
The transaxle works by pumping oil through a series of valves that create a circular drive motion that is then transmitted to the wheels. See the parts
site for a look at the transaxle internal.
As oil gets hot its viscosity drops, essentially making it "thinner" so it has a harder time sealing in the valves and creating the drive to the wheels. And, as mentioned, the more times the oil is heated the more damaged it becomes and the less resistant it becomes to heat and load.
Since you have the transaxle out you can check for any bluing on the parts inside or any debris in the oil to suggest a problem with overheating or a failed part. If nothing obvious is found, you could try refilling the transaxle with fresh oil to see if that improves matters.
If not, then you may have damaged pump or valves components that need to be replaced. At that stage you may want to consult a professional or even consider replacing the complete transaxle. You can get more information on professional services in your particular area by calling
As noted in your
this is a Hydro-Gear 319-0650 transaxle. The Hydro Gear 310-0650 transaxle uses 20W50 oil. It holds approximately 80 ounces of fluid and the fluid level should measure 1 1/4" to 1 1/2" below the top of the housing when full.
There is additional information on checking belts issues and other related maintenance in your manual and in the