Ours is a 2007 or 2008
M59 too. One of the first.
Beneath the floorboard there is a stranded control wire - very much like a bicycle/motorcycle cable - that runs from beneath your HST pedal rearwards to wrap around a spring-loaded drum about foot to the rear. That cable is sealed so that no dirt can get in and should last forever....but it doesn't. I replaced mine a few years back and posted here... I'll try to find that old posting so we can get the part number.
Anyway what you are feeling with yours sure sounds like what mine did right before the cable broke. Basically a few of the strands had parted so that the cable wasn't winding around the spring-loaded drum equally in fwd and reverse. Kind of hard to explain, but I bet if you lay under that side you can see the control wire. Then it makes sense.
It is hard to access, but you can do it with strong fingers and only a few tools. Be careful to run the new cable exactly like the old one. It took me about an hour of futzing to take the old one off and replace it. And then another 30 minutes to convince myself that it was really fixed... it was...
I just checked, the post was:
https://www.tractorbynet.com/forums...09440-m59-breakdown-repair.html?highlight=M59
Not knowing how many needed replacing, and since Messick's shipping is expensive, I ordered all 3 cables from Messick's, I believe the one that got used was the KU 32791 - 44730 It was about $30.00
I also bought a 32791-44750 and a 32791-54790 cable, but they were fine & now live in my spares box.
rScotty