Hi,
Do you know someone who's into electronic repair? Or have a local TV repair place? They could check the CDI over for any dry joints - something ECUs in cars sometimes suffer from, yet most get replaced at a cost of hundreds when they could be repaired. If you were in the UK, I'd happily do it for you if it were posted to me.
Something that was affecting the idling on one our Mule 3000s quite a bit was the fuel pump. When I dismantled it I found the contact points were in bad shape and one was worn down terribly. Some thick copper from some large fuse's legs (300A) ground down, soldered on and filed to the right shape made it as good as knew. Almost 2 months on and it's still fine (incidentally, did the same with the contact plates in the starter motor after it died and we just got a click when trying to start it, that was 4 months ago and still working great).
Other than that, excessively high or low compression, dirty fuel filters... can't think of anything else.