You need to jack up mower to get rear wheels off ground. Disengage free-wheel handles. Set parking brake and start mower. Once engine is running, move handles forward and hold for 5 seconds, then move aft and hold for 5 seconds. Repeat this 5 times. Shut mower down, remove mower from jacks, re-engage free-wheel handles. Start mower, drive forward about 5 feet, turn and drive back 5 feet. Repeat this 5 times.. That should clear any air out of system.
Try purging hydrostatic system. I just did that and now works fine.
I have the exact same issue. Mower was running fine. Works on level ground and downhill. Then when climbing slopes, it barely climbs or even stops.