I have a 1994 C280 that would fail to start in the small time window between a cold start and a hot start. When it had only been run for a few minutes, then shut off, it would crank over and over but never light up. After sitting 5-10 minutes it would always start again. My MB dealer did a check of the "fuel pump pressure" and tested the "generator with regulator and cables" (all this according to the invoice). They determined the problem to be the "overload voltage relay module" (part # 000-540-67-45) and replaced it. The problem hasn't happened since. The part was $119 Cdn and they charged me .2 hrs of labour to install. Hopefully this saves you the cost of the diagnostic tests.

Jonathan