Originally Posted by Chalk Two

It might help, but at 3500 rpm you may well be in the area where the fuelling is switching from 'closed loop' to 'open loop'

If you ask Kev in the 'ask Kev' section he will be able to explain what's happening here, especially if you're experiencing 'lurching' or 'surging' at, or near, that range. Changing gearing will just move that surging to a different point in the rev range. I would definitely recommend the O2 mod if this is the case.