i have a ls400 1992 and the revs idle spot on at 650 rpm but when i gently press the gas pedal slowly increase it to 1100 it is fine same at 1400 and 1600 but as soon it touches at 1800 & 2000 rpm the rev needle bounces down and up all the time but if i increase the revs to 2200 onwards it is fine it is always on 1800 & 2000 revs.please can some one give me an idea on how to sort this out, i have cleaned the throttle body and checked the vacumn pipes they are not cracked.
much appreciated