Tick over on my car is not good it seems to miss

tick over on my car is not good it seems to miss once every few seconds. car exelarates better at 3.5 rpm. when i stop the mpg gage rises to 30mpg gragualt when stoped then drops to zero also when driving the mpg gauge is verinaccurate to normal ie when it should be 40mpg it will read 10mpg then drop to correct reading ( i know the gage is not accurade but its flucuatin reading tell me that there could be a problem finaly once i was stuck in a traffic jame for 15 min and the car started to miss fire and drove with no power as if on three cilinder once I switch off ignition and started again it was ok. I have just replaced the air induction pipe to the manifold and idle vale. I have a e46 1999 328 petrol