Fuel Supply Problem 1995 E 300 D
 

I'm thinking of buying a 1995 E 300 D estate that has a fuel supply problem. Around town at relatively low revs their does not appear to be a problem. Once on the motorway at high speed and hi revs the fuel supply apears to breakdown causing the car to misfire. Anybody any idea what might be causing this problem, the fuel pump has been replaced and injectors tested. Should I steer clear ??