My machine's HST is drive-by-wire. Two things I'd look at are the foot pedal input and the servo output and the wiring in between. Get yourself the WSM to help you diagnose.
I checked the voltage coming into the shuttle lever sensor, it the 5 volt spec. When I checked the resistance on the sensor, it was out of range. It read 1.12 and spec is not over 1. Could this be a cause of the issue?
I suspect it's the forward clutch pack having an issue-but the control system for the packs can also cause the same end result. Thus, a good thorough check of the entire system is in order. Hopefully it's simple and inexpensive.