Outer Limits 4x4 Board

I have a 1997 FZJ80 Series automatic LandCruiser with 300,000 kilometers on it. I have just had the valve shims done and the injectors cleaned, new plugs and air filter. I recently had the old vapor LPG system changed to a vapor sequentially injected LPG system. The performance difference is remarkable, as much if not more power than on petrol. Recently however it has developed a strange intermittent shuddering that can best be described as like driving on a corrugated dirt road, the steering wheel and dash shake quite violently. It only happens on a very light throttle. The moment I lift the foot or press a bit harder it stops. It happens at any speed from 60kph up to 100kph on either fuel so it's not fuel related. I have a vague recollection of reading something about a throttle position switch causing something like this but I can't find the thread. Anyone got any ideas what is causing this juddering?

Regards,
Alan.