Pierburg intermediate high idle speed

Hi all, need some advise on my clipper cabby. Sometimes when I start it the revs stay at around 3000rpm all the time at idle if I drive it till its warm it also runs on when you try and turn it off. If you go over a bump tho it will randomly idle perfect and be fine then until next time. Had a look at the carb and the vacuum diaphragm on the drivers side of the carb 3/4 point unit???? is pushing the throttle lever forward to make it idle high, sometimes however it seems fine. Any ideas???? Thanks

have a read of the 2e2 faq thread, check everything top to bottom