I have a 1989 ford yt16h hydrostatic mower and it wants to pull when you first start it but just quits pulling all together when it warms up,it is full of fluid,it does have a filter on it that might need changed.

Did you check that the belt is tight and not slipping on the top of the hydro ? Also I have a website of expert on these Ford lwan and garden tractors to recommend. Check out Jacobsen_Ford_Garden_Tractors : The Best Jacobsen/Ford Group On Yahoo
I know there are plenty of people there with the same tractor that can help.

Good luck
Morrowrj