1450 #1, another hydro problem.

[cdlong]

Last week while mowing, I noticed it was slowing down more than usual, ( the longer I operated it, the slower it went), going up the 2 grades from my neighbor's yard to my yard. I also noticed the tractor was bucking, (almost stop, then resume speed), while backing down her driveway. Didn't really think much about it until this week. I use the lock on the shifter to maintain the same ground speeds as I mow several of the retired neighbor's yards. This week, while mowing my other neighbor's flat yard, the tractor was taking longer to get up to speed. Then when mowing my yard with the grades, I had to move the lock on the shifter, and move the handle further up to climb the grades. This tractor also picks up speed when going downhill. Enough speed that I have to pull down on the shifter and slow it down. I had changed the hydro oil at the beginning of the season because the rear cover plate gasket was leaking. I checked the oil level and it is correct and clear. I think the filter was changed 2 years ago when I put new springs and keepers in the trunion. Neutral is perfect, no tire movement. Any ideas what I need to look at? I intend to pull the fender pan this weekend and check the trunion.