hesitation and lack of power

[Tornado-red_gti]

I have a 1990 mk2 8v gti with a very annoying occasional fault. When I drive the car then stop, to get petrol or whatever, then get back in the car and set off, it hesitates under acceleration and has an almost complete lack of power!
But if I stop again and turn of the ignition it nearly always goes back to normal.
Has any one got any ideas what it could be?
Cheers.
        Dave

[golfvr6]

Since it is occasional, i suspect an electrical fault. Maybe a faulty knock sensor.

[Tornado-red_gti]

OK, nice one. I cant seem to find it in the haynes though?
Could you point a complete novice mechanic in the right direction please.
 Thank you
      Dave

[golfvr6]

The best thing to do for starters is get the car on a fault reader.