Hi ive noticed afew probs with Gti's only reving upto 5000rpm...
mine was the same just thought id post up how i cured mine, may not be the same for all but most likely it will be..!
My Gti 8v was only reving to 5000rpm, so first of all i had the timing checked (cost £40) it did run better as mine was 4 teeth out

and then had my dizzy adjusted with my cap (£10) and rotor arm replaced (£5).
This sorted the prob out straight away... so if you have the same prob, try them 2 solutions out first before spending big money..!
Cheers.
