I have had the exact same problem. If I let the car sit for a while, then start it up, I'll notice somewhere down the road a power issue (usually at a certain spot in the throttle). If I pull over though, the problem WON'T go away... I must stop the engine, wait for 5 seconds or so, and restart it. This makes it all go away for (as I've noticed at least) the end of the trip.

*MY* conclusion is that it was either a fuel problem, air problem, or the computer was finally trying to kill me. (Open the pod bay doors, Hal.)

However, someone who would know indicated it is likely a fuel problem, which does make sense if you think about crud building up in the filter flying up on a startup where the engine temprature has changed. I may be full of it. I'm sporting a B280F engine, so who knows what kind of gremlins are about to come rewire my stop lights.
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89 760 GLE 150k/ 86 740 GLE 240k -- Still going.