Mitsubishi Repair: Mitsubishi high rev irregularities, throttle position sensor, frequent occurrence

Question
Hi there i recently purchased a magna altera 1998 tf model, and up until now all i have had to do is replace the key reader and starter motor but currently and its not always the car will start and will rev at am enormously high rate e.g: instead of starting at 1000 rev rate and dropping to 750-500 when changed into gear, it now will start and kick up to 2000 rev's and roughly stay there and when is in gear it will sometimes (even just rolling no faster than 10k/mh) will jump up to 1500 rev's then dip down.this doesn't always happen but has become more of a frequent occurrence i have read that its a good idea to change the transmission oil and filter.do u suggest this?and what should i do?
Thankyou for your time. Regards Nathan

Answer
Nathan,
It's always a good idea to change all your fluids per the manufacturer's schedule.  A bouncing idle can be caused by a few things.  The most common are a malfunctioning throttle position sensor (TPS), a bad base idle set screw (BISS), or a faulty idle speed control (ISC) motor.  Usually a bad TPS would cause your check engine light to turn on, but a bad BISS or ISC would not and should be inspected by your local mechanic to ensure they are operating normally.
Good luck!