it could be nothing more than a blocked fuel filter ! or if the smoke is blue and the engine rattles it may be a build up of oil in the intercooler then blowing through the engine under full load.

If the engine is litteraly bowing out at 3500 it may be over boosting and dropping into limp mode.

I would concentrate on fuel filter first and if you replace it its advisable to strip the old filter and make sure its not covered in a black slimey film ! if it is the tank has got a bacteria in it!

Many thanks Injection doc. I'll take a look at the fuel filter but if it isn't that I'll probably try and learn to live round the problem -- no serious acceleration. I quite fancy your second theory though in which case I'll definitely have to learn to live with it -- sounds as though it could be a bit expensive to rectify. The same goes for overboosting.