Erratic throttle control

Hi,
I have a BMW Z3 2.8 1998 Roadster. I have an intermitant problem that at first seemed like a sticking throttle. When I am cruising on the motorway sometimes when I take my foot of the gas the engine wont decelerate and continues at that speed. Sometimes if stab the throttle pedal again it will release other times it will stay open at the new position. Then other times the Gremlin takes over and the car starts to accelerate without me touching a thing! When accelerating hard one time the engine continued straight up to 6000rpm after I took my foot off the gas and stayed there until I turned off the ignition. When I restarted after 10 mins everthing was OK? I am sure its not sticking as it revs on its own accord! A diagnostics check shows no faults!