I agree with RBM...
The problem is with the engineering of the system. I used to work for Philips Lighting on the HID systems for GM products and supplied bulbs to BOSCHE. Our biggest problems were with the ballasts and other electrical gremlins. The 'bulbs' are maufactured and tested to very strict quality control. It is very doubtful that the problem will be found in the bulbs.
If you are going to have the dealer go to work on the HID system, have them replace the ballasts and other electrical components first.... These are the only reason for poor light operation/output. My guess is the supplier of electrical components will provide your final solution, in engineering improvements.
76 240D - 545K miles
78 300D - 200K+ miles