I have '84 300SD with EXACT same symptoms. I phoned Peter? (at GDL) and he indicated CC ECU was the culprit and said to send it infor refurb. I may try the circuit board check and resolder route first. If anyone has had a "cure" for the "restart to reset" problem I'd be interested as well. Thanks.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)