I have a intermittent fault with my engine, It will suddenly lose power and i have to rev it quite high to be able to keep moving when it starts to do the problem it will idle erratically nearly cutting out but then revving up to 2500rpm and back down to 300 ish. if i stop the engine and restart it will be fine for a while, it could be 300 miles or 3 miles before it does it again. the engine also seems to run very unevenly sending vibration into the car and some black smoke and liquid comes from the exhaust (only when doing the problem though) but the rest of the time runs smoothly and clean. I have had the car plugged in at a main dealer and there are no faults logged. It seems to do it more when the fuel tank is below 1/2. I have been told it might be the MAF sensor??