Unfortunately the bucking did not stop, it got worse. I asked for advice again, and once more the mechanic recommended repairing the clutch. Originally I had been planning to buy a new car in the second half of 2007, so that would probably have meant to prepone that purchase. So I started skimming through car sales notes already...
In a final attempt, I invoked a Google search on my car's model and construction generation, as well as several keywords describing the behavior. Hundreds of online forum posts showed up. I took a look at the first results which included possible explanations, and did a quick count on which parts might be involved. Seven out of ten postings listed "exhaust gas regulation valve" (I hope that's the term in in English), others such as "lambda probe", "fuel filter", "spark plugs" or "injector" scored between one and four (some articles specified several causes).
At this point I should probably mention that I have absolutely no clue about cars in general, and just a very rough idea what an "exhaust gas regulation valve" might even be. But I went back to my repair shop and told them to give the exhaust gas regulation valve thingy a shot instead of fiddling with the clutch. They did, and sure enough the valve turned out to be the root cause.
Thank you Google!