I have a 1999 LHS that is reving up about 1,000 rpms while you are driving. It does it even if you take your foot off the gas. it does it about once ever 100 miles. Have checked for codes, but none show up.It does not seem to be the transmission.
Any ideas!!! thanks!!!

 

could be the throttle position sensor, sometimes these go goofy and won't produce a fault code, this usually makes the transmission shift weird as well. could also be a vacuum leak but since the problem is not constant I would suspectit's a sensor sending the wrong info from time to time. A friend of mine was pulling his hair out with his Concorde sometimes it wouldn't start when hot. no fault codes, new fuel pump and some of the other usual suspects, ended up being his camshaft sensor.

 

does sound like a vacuum leak,i would spray carb cleaner around the intake area.it idle speeds up when you spray a small area then you've pinpointed your leak.common on pass side rear of engines of that year.moe