I have a 1978 Mercedes Benz 300CD (diesel) that is losing power during my commute to and from work. After being on the freeway for twenty minutes the engine starts to feel like it's struggling and starving. If I slow down to forty, it's fine. However, when I get back up to cruising speed (between fifty-five and sixty) it starts to struggle again. Why? Just note, I've checked the fuel and air filters and the vacuum pump recently gave out (no power assist on the brakes and the engine won't shut itself off without manually "choking" it), which I have to rebuild (I've ordered the kit). Is it possible that it's water condensation? It's been bitterly cold here, recently.