280GE - random cutting out

Hi, asking this on behalf of my father who has a 280GE (H-reg - 91?). He has a problem where it keeps cutting out and stopping, did it the other day 3 times on a trip of approx 8 miles. I appreciate I will need to give you guys some more info but just wanted to get the ball rolling. Vehicle has what appears to me to be an aftermarket immobiliser on the dash to right of steering wheel, kinda like a small black plastic two pin socket. To start he has to briefly connect a matching 'plug' to this connector before he is able to start engine (from memory, on the occasions I have driven it the engine will crank but not start unless this routine has been done). Local MB dealer claims to have eliminated this as source of problem and has also recently changed the ignition barrel (or was it door locks?) in an attempt at fix. Still doing it.
Oddly enough he had a very similar thing with his previous G, which niether he nor the dealer ever sorted. Freom this I'm wondering if it's a common problem with a simple solution.

TimP