My 3130 HST Owner's Manual has--memory here--a brief section about how to confirm all safety switches are working correctly: sit down, do this, push that in a particular sequence. It ends with 'see dealer', but at least you'd have an idea where to start looking.
Different machine with shuttle like yours has a microswitch mounted alongside the base of shuttle lever under the dash plastic. Get to yours and figure out if something changed orientation or has gone bad. For whatever it's worth mine is the third GST I've seen where the shuttle safety switch had apparently failed and been removed from service by [Previous] Owner(s).
The only other thing is start tracing wiring and finding all the safety switches. Our 3130 has about as many hours as yours and has been flawless; I've never had to search out all the interlocks.
Could be PTO (electric handle partially engaged effecting switch), seat switches, or even somehow fuel solenoid wiring. Think simple and work your way toward increasing difficulity and maybe you get lucky.
Messick's can furnish wiring diagram from parts book. Check also KubotaBooks.Com and Kubota website.
Please post back with remedy for all to learn.
