I haven't ever driven another properly running OM603 car, but it seems to accelerate from a stop with no problem, and have enough power to drive at 65 mph on the freeway, but both at freeway speed and in town, if you have to slow down and accelerate again, it will not gain any speed with the accelerator to the floor until it eventually downshifts, and then will very very slowly accelerate to 3000 rpm, then accelerate normally after that.

I haven't driven it much since I bought it and I think it was like this before but it makes it so its not really safe to drive in traffic on the freeway.

Any ideas what might cause this or what to check? It seems to run fine the rest of the time.

I don't really know much of the history of it, it looks to have had the injection pump replaced at some point but not by the guy I bought it from who hadn't really done anything except chop the springs and ruin the interior (which I have since fixed).