My wife's 2001 was doing the same thing, revving to around 2000 for no reason. Turns out it was a combination of bad gas and wrong O2 sensor. After July 2000 Isuzu changed the O2 sensors. With an older, 98-2000, sensor in there it goes bananas and does not know what to think.

'94, and '00 Amigos and '00, 01 Rodeos. Rodeo springs and aftermarket 'hard top on 94. Aisin hubs OME 919's on the 2000 Amigo. 2000 and 2001 Rodeos have OME 912's and Aisin. Smoother ride with the OME 919's???