When the car is warm unscrew your oilcap, if the revs drop away down its piston rings or worn bores. Its probably just valve seals to be honest which arnt a big deal until your using more oil than u can afford. A replacement engine can be git very cheap like £80 cheap. Takeoff your airbox pipe, hold open your throttlebody and using a cloth and your finger give it a clean inside. Start it up and see does it idle any better, if not clean it abit more. Then while its still running unplug the idle valve which is on the bottom of your throttlebody. If it revs up then your idlevalve is clean and working-plug it back in. Next the black black hose from your rocker cover to inlet manifold is the breather pipe.