There was another forum member earlier this year going through the same issues with a diesel W201. He swapped in something like 2 or 3 different PBU's including a programma-rebuilt one. His issue ended up being poor solder joints on the PBU, despite going through several.

Inspection is visual. The solder tends to crack around parts with mass, so inspect around the pushbuttons first. If you see a "ring" around the solder pad it's broken and needs resoldering.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]