I've had my 83 300SD ~ a month. When I got it, the cruise was working but
had terrible surge/retreat - unusable unless you like nausea.

I tried what Brandon said:
"you might just try hitting up the connector end of the unit and doing those solder joints"

A couple of those end joints looked flaky and I heated them. Also, I took a brass brush to the pins and female connectors hanging under the steering column.

Now, it is very usable...only a hint of surge. I'm quite pleased with this "limited" repair...if I took the soldering iron to the whole pcb, THERE WOULD BE some cut ray tracings.