(WOT) in first or second gear the car stumbles and/or hesitates. It starts to shake, the revs starts to bounce and it won't accelerate at all, I then have to take my foot of the accelerator let the revs come down a bit and gently press it again to get going. As soon as it hits 4000 rpm I can put my foot down again.If I drive the car normally there's no problem at all, it's just between 1 and 4k rpm if I accelerate hard.
Plugs, leads and O2 sensor are new
(2000 km) and the timing was set with a timing light. What else could be the problem?