Doesn't do it all the time. Might be 4 or 5 times a week, but sometimes I get a surging like idle.

You can hear it surging a little and the rev counter is gently twitching up and then back down to normal. It seems to be adding about 30rpm. I thought lpg mix to start with but then I checked the petrol and it still seemed to do it. Might the IAC valve be the culprit?

They do get gunked up. Won't hurt to pop it off and give it a blast through with carb cleaner so the spool inside moves freely when you shake the unit gently on its axix.

Had it off a while back when I stripped top end. Cleaned it all up then. Doesn't appear to be knackered. Rattles freely.