My t reg almera kept dropping revs when clutch in and coasting, was always threatening to stall and the rev counter spent alot of time hunting around. I'm happy to say it's all fixed now and required a very easy solution: Take off your air filter housing, buy some carb cleaner spray (I used some Holts) then blast down into the throttle body, whilst opening and closing the butterfly valve (you can move the accelerator cable by hand to do this), leave it for a minute and start your engine, there will be some white smoke but all should be well with regards to revs (unless you have another problem, AAC valves are a common one and need a clean, but they're a sod to unscrew). I repeated this a couple of times, and you can use an old clean toothbrush to scrub the gunk out, but be very careful with the air intake area, don't let anything fall into it, be gentle with it and avoid those two little transistor type things at the top, they're an expensive fix, I also kept the cleaner away from these. After all this I got a KN filter, but cleaning out the air box and hoses plus a new filter will also help.
Hope this works for you and anyone else who has this problem - I blame it all on the crank case ventilation hose which pumps oil vapour into the air intake for environmental reasons, the standard air intake has two filters before this enters the engine, but amazingly the kn cone replacement filter just has it passing directly into the engine, which would surely gunk up the throttle body quite quickly. All this used to happen to my old golf roughly every 6 months, the ventilation hose was the culprit here too.
