Small Engines (Lawn Mowers, etc.): Hydro Gear Transaxle, exact model number, summer temps

Question
I have a Craftsman DLT2000 25HP(B&S) 48" mower, Model #217.272263.  Hydro Gear Transaxle Model # 323-0510, Part # 173839.
The problem is lack of pulling power.  Engine runs fine.  It gets worse the longer it's run, at least in the hot summer temps.  Forward is bad; reverse is worse.
I've replaced belts and the oil in the transaxle.  The oil helped considerably.
is there a part/parts I can order and replace to fix the problem?

Thanks
Tom

Answer
It sounds like your charge pump.  Have you checked the tow valve?  It is the lever you move so you can push the tractor.  If slightly off it will cause this too.  If not, there are parts available.  But first I would price a replacement.  If the repair costs more than 1/2 the price of replacement replace the transmission.  

If it is a Hodro-Gear or a Peerless there should be a seal kit for it.  This alone will help.  There are service manuals for these transmissions from the manufacturer.  Hodro-Geatr is in Sullivan, IL and will take calls from customers but you have to but parts from a dealer.  www.searspartsdirect.com is the first place to look. They have a good search feature if you have the exact model number.