I'd do as jon@work's excellent suggestion first, fuel filter, could be all manner of things, check the crank position connector doing the wiggle test to see if alters.
When I first got my car it had been owned by two cossie "specialists" but still had a 12 year old fuel filter, there was literally no power below 2500, so little I couldn't change into 5th if below around 3500 as it would bog down unless downhill, nothing showed on rollers as wrong, outright power was ok and correct for spec, after filter change was a different animal pulling from 1500 in top without problem, no faster outright but so much better, so it's not just outright power it affects.
tabetha