Lately i've noticed that my engine idles bang on 1000rpm. This isn't even after a cold start, it just happens randomly (maybe once a week if that). Normally its around 800 rpms and bounces up and down slightly, but when it idles at 1000rpm the needle sits there solid, doesn't move at all like it does when it idles normally. The problem seems to stay the length of a journey and goes away after i've switched the engine off. It doesn't affect the way the car drives, but just seems strange. Any ideas?