My Kubota T1600 loses power when it warms up?

My 1994 T1600 has a 14 hp diesel. After it warms up it looses all power. It does not matter whether the blades are engaged or not. It will pull a hill just fine when it is cold, as soon as it warms up it will not pull the hill. The engine seems to bog down so much that it almost kills the tractor. It has is hydro-static drive. I have ruled this out as it works just fine cold. Just looses power after warm up. Any help would be greatly appreciated. Point me to another site or just help me here. My email addy is dishlowry@yahoo.com thank you.

John Lowry