I bet your over-voltage protection relay is dying. My car had the same symptoms till the abs-tach-electronic idle wouldn't come alive anymore.

I went to a junk yard bought another one and it works just fine now.

__________________
1992 Mercedes 190D 2.5 turbo 5sp manual. EGT+boost gauges. Boost controller set to ~14.5 psi. 1 1/4 turns on full load adjustment. LPG injection. Next in the list is water injection.