Failing to disengage engine

Hello,
I have a T520 Gator and it has a reoccurring issue.
when i attempt to stop/slow down by taking my foot of the accelerator the engine doesn't disengage and tries to power forward- usually ends up stalling when the foot break goes on or occasionally calms down, it is not always an issue as it comes and goes- but when your in front of a lake, wall or tree you need to stop!
- i believe it could be a damaged gear box?? which isn't allowing the gears to disengage?

Thanks