I found this interesting because I've had this with my K66 (very similiar to your K71/K72). A simple problem with a little time.
This sounds like the problem "creeps in neutral".
If when you experience the creep, you apply some reverse pedal input just to see if it stops and then stays stopped, then this is an indicator of creep. (you are manually returning the control lever to neutral)
When you start the tractor, if you have not applied control, will it stay, or creep?
Have you lost power in one or both directions? If no, its most likely creep.
Three categories that can cause creep,
1) Speed control linkage, bent, or obstructed by built up debris.
2) The actual lever on the transaxle or roll pin being loose or damaged, play.
3) Internal failure of the transaxle.
Of these three, the first one is most common and one that I have had. Even a bad nylon bushing can contribute to creep, or grass clippings built up around the control arm.
The PDF is Tuff Torq's Trouble Shooting Guide to their transaxles.
Because your chassis is a very strong chassis, I doubt you have any flex or horizontal torsion from front to back when on extreme uneven terrain, so ruling this out as a culprit to common grass cutter chassis's, I would look from your pedals to the control rod, to the transaxle for anything that is damaged, loose, alignment issues, or binding on the return of the control rod. I believe you will find your answer there.
You can rule out the shock by disconnecting it, just for the purpose of ruling it out. The shock was designed to dampen the response from motion to stop. (customer concerns on a stopping jolt or abrupt harsh stop caused the addition of the shock according to Tuff Torq)
I hope this helps.