Problem is with a 1993 518i, recent replacement for previous 518 (it had a run in with an idiot driver and didn't survive, my dad driving it did though so thank God for the BMW's superb crumple zone). The engine goes rough and will not rev above 4,500 to 5,000 revs. This is only under load and it will rev above 5k while in neutral. Spark plugs & leads, air & fuel filters have been changed.

Anyone have an idea what might be causing this? In the UK if this makes a difference.

fuel: fuel pump, fpr, injectors, bad gas (petrol)
electrical: crank sensors, AFM, tps,