I think you are getting to the core of your problem...the carbon! I am not sure that I would run the hotter plugs for too long, as they tend to be better if your engine is tired and letting oil in that needs to be burned. You have low mileage and I bet are not using much in the way of oil. You actually have higher compression because the chambers are clogged up with the carbon.

I would run some alcohol for a while, and for sure on your trip to Germany. Another old method for getting rid of carbon or keeping it from forming, is running a water injection system. For example, many WW II Fighter Planes ran a water injection system to keep the carbon off, and power up. It has the additional advantage of lowering the combustion temperature too, which can have the effect of feeling like you are using a higher octane fuel.

There are fancy systems that you can buy for race cars, but you really do not need one. You do not actually pump water in, but instead water vapor or a very light mist. A basic system is a jar under the hood with water in it. To this you connect a line running to the jar with a little valve to adjust the air flow, and an exit line from the lid to the intake. You end up with bubbles and very moist air...and it's the moist air that gets drawn into the motor by using it's vacuum. I'll see if I can find some more on this for you. I have never seen this on a Volvo, but do not see why it would not work, but I doubt you really need to go to all this effort to fix your problem.

You can also just use a spray bottle with either water or a mix with alcohol in it and squirt...really mist...the contents into the intake with the car running. This can be very satisfying because you can see the the carbon come out the tail pipe. Make sure you do not have the car with the exhaust facing your garage door...as I once did...or you will have a big black carbon mess on the door to explain to your Wife! I bet your 240 has left some black carbon soot marks on your driveway when the weather was just right...really high humidity, but not raining...(if it were raining you would not see the marks)...because the "right" weather conditions can create a sort of "natural" water injection system.

If you are really plugged up, or want fast results, you might remove the head as Jorn suggested. However, if you have had it up to speed on the Autobahn, I'll bet you are not that bad off. One major indicator of the need to pull the head, is if once you get out on the Highway, the car just will not get up to any speed. My Grandmothers car I mentioned would not go over about 30 mph anymore, and it wanted to die as the thick black clouds poured out.

I think in the end, if you go the slow route, by adding some alcohol to your tank for a few tankfulls, you will get to the problem. Driving will be the best cure and future prevention.

Good Luck!