Wow .... I'm not sure. From your description is sounds like the problem is somewhat intermittant ...... so I doubt it is a fuse. Then again the modern fuse box on the W210 models virtually eliminates the fuse problems of eariler models .... so if your 1995 has the old style fuse box with round style fuses then perhaps checking the fuses and cleaning them might be worth a try. When you swap the two valves does the intake flap work all the time? If it does then I would consider taking a chance and purchasing a new valve because electronic issues are difficult to track down and it is possible that the valve is intermittantly bad either on the vacuum side or the electronic side. If swapping the valves doesn't correct the problem for the intake flap operation then the problem is probably electronic. The problem could either be in the control wire(s) from the ECU to the valve, or it could be the ECU itself, or it could be a bad sensor or wiring connecting one of the sensors to the ECU. I don't know what sensors drive the movement of the intake flap via the ECU. Trouble shooting such a problem requires a manual and wiring diagram to do it right. Good luck.

David Roseman
Long Beach, Ca