Got a 740 1990 B200 engine. On an intermittent basis, although the frequency is increasing, the car starts o.k., but then runs rough with the RPM varying significantly. The throttle has little effect, and there is no power. I have replaced the fuel pump relay, recommended by the breakdown man, and both pumps seem to working.
Previously, when the car was running, at about 60mph, there were a couple of "coughs" or "hick-ups". When this happened I noticed the RPM dropped to around 500 to 1000 RPm, but the engine was still in gear, and so was turning over at the same speed as the road. Not sure if this is significant ?
A suggestion from another breakdown man, was that the fuel pipe in the tank may have split and so the fule pump would be sucking air into the line along with the fuel. Does this make sense ? The tank is less than 1/4 full, but I put another gallon in from a petrol can with the engine running and this made no difference. ( I thought if there was a split pipe more fuel could cover the split and stop air getting into the pipe )

After the two times I have needed a tow home, the car has started and run fine.