serial HID control unit failures

[enotslim]

I'm looking for advice about why my headlight control units keep failing. 2004 XC70 with Xenon headlights

Driver's side headlight was out for a few weeks. Broken bulb.

Replaced bulb both sides with OEM (fcp HID Jahn D2R). Both worked when connected to the igniter removed from the headlight assembly. After reassembly the driver side bulb was out (passenger side was good). Swapped parts to determine the driver side control unit had failed. All other parts were OK including both new bulbs.

Replaced driver side control unit (fcp Valeo 30727205). Both sides worked when fully assembled in car, great. Rechecked several times with success.

An hour later I drove 1 mile to inspection station. Passenger side bulb was out. (Passed inspection anyway.) I swapped parts to determine the passenger side control unit had failed.

Before replacing the passenger side control unit - is this an unlucky sequence of events (plus old parts that tend to fail at the same time) or is there some underlying electrical issue causing serial failures? Should both control units be replaced when either fails for some reason having to do with upstream electrical control? Does a new bulb cause the control unit to fail?

Thanks.