With engine off, look up under footrest where the forward/reverse pedal is located - move the pedal by hand back and forth while judging it's tendency to stick or "slop around" in the pivot shaft bushings. May need a shot of dry lubricant (avoid oil/grease) or possibly bushing replacement. If the pedal is hanging/binding slightly, there can be some tranny creep.
Also chk balance of linkage back to hydro tranny fo similar probs.

Also keep engine running full RPM when cutting to avoid excessive heat buildup in hydro oil.

Grass strip left at high cut settings - would first chk/adjust front to back deck leveling while at that cut height setting. if uncut strip only happens during a turn, may just be the nature of the beast. New blades - installed correctly - not worn at their tips - may help.

Thanks rcbe,Since I posted,have jacked up mower and the pedals and linkage work ok and are not under any bind and are like new(only 70 hours). While moving it to jack it up, hit brake a few times and that made it come to a quick stop,when changing directions.(didnt hit brake while changing directions) As I know nothing about a hydro,am wondering if they always creep a bit and the brake stopes it,even with out applying it. Did find a small leave stuck in the brake system,but not directly on the brake disc,so blew all dust and grass of area. Working good now,but was also working before I jacked it up.
The blades looked like they have been thru civil war,so will order new ones. Even when it was new,it left a few blades of grass in middle when cutting high.