2005 Seat Ibiza engine speed sensor... ??

I was driving my car and all off a sudden the revs dropped and the engine diagnosis light came on and for the rest of the journey i couldnt rev over 3000 revs.
I hooked the car up to the diagnostic computer thingy and it read that i had a fault with my engine speed sensor. I looked it up and read that it acts like a magnet and attracts dirt and metal shavings, and that it could be as simple as removing it and giving it a clean.so after finding and struggling to get access we got the sensor out and yes it was very dirty, gave it a clean and put it back. turned the key, the diagnosis light went out and the car reved like a dream. that day no problems.. next day after maybe 16miles it did it again but only through two gears then was ok... it went two journeys with no problems and then it staretd again and been like that now for nearly a week. its an intimitant fault and its driving me crazy. any ideas?? the sensor dosnt look damaged.