random engine revs,wierd idle

[240sx1234]

looked at the search,but nothing i see to really answer my question
i have a 95 240sx se (stock ka24de) and hks exhaust only things ive done to the car,when the car is just sitting by itself it will randomly start to rev up and down.not to high like maybe 1500 at most then back down,it does it when i turn the wheel and turn the heat on which draws more power...i know.but sometimes it will just do it sitting there without touching it.if you need any more info to help me fix my problem let me know.it just feels like the rpms go so low and vibrates a lil and feels like its about to stall (5 spd btw)