Small Engines (Lawn Mowers, etc.): Hydro transmission, air bubbles, swash plate

Question
I have a John Deere stx 38 with a black deck.  The engine starts and the tractor goes fine for about 1/2 hour. then when I put the Hydro lever to forward on slight inclines it goes real slow some times it does not go.  If it is flat it works ok not as fast as it used to be?  Any suggestion?  Thank You.

Answer
Run the engine at wide open throttle all the time.  

A good test is to put the tractor against a wall and run the engine at full speed.  Engage the transmission and the wheels should break traction.  I mean spin.  If they do not is the fluid clean?  Is it full of air bubbles?  If so there is an air leak.  

You didn't say how many hours were on the transmission.  If allot it could be wear on the swash plate.  If its the transmission take it to a dealer for repair or replace the transmission.  You could try to repair it but it is less hassle to replace.  You could be putting it in and out a dew time if you don't know what you are doing.  John Deere should have a service manual on this too.  If they don't get the model of the transmission and go to the transmission manufacturers web site and order a manual.  Good luck on your project.