I bought a used Toro Z master Model no. 74262 that struggled to go up small inclines. Not stuttering, but it would make a groaning noise and eventually blew out the shaft seal on the pump. Found metal shavings in the filter. Repaired the hydro pump, flushed the fluid and fixed the problem.
Repair involved removing the pump, grinding the face of the aluminum endcap to remove the grooves where the pistons ride. (and replacing the seal).
My other pump did the same thing. The pump was different so had to buy a endcap kit for $200 (came with the piston block assembly). Couldn't polish the same area as first because the endcap casting was different. (had a cast lip and offset surfaces, don't know why they changed the design). I thought fixing the pump was easy compared to removing it from the mower.
Anyway repaired the pumps and my $900, 2006 Toro Z master runs great.