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.