My 93 300D developed a surging voltage problem recently. It was bad enough that at night, you could clearly see the headlights pulsating on the road, and the radio display looked like it was playing disco.

Replaced the alternator voltage regulator - $35 and 15 minutes (three minutes if it's already on the ramps). Two screws and it pops out from the back of the alternator. Installation is the reverse of removal.

All is well now. Love the cheap (relatively) and easy fixes.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

i had the same problem and used the same fix... im very glad the regulator isn't built in to the alternator like on some American cars