Could be (but not sure) the DK motors. Maybe you first clean them and test again. Info on my website below. Done in 1-2 hours.
Or the fuel pressure is too low or fuel runs back into the tank due to a bad check valve. Engine room hot = air/fuel bubbles could build up
http://www.bmwe34.net/e34main/trouble/535ihot_start.asp
Try: switch ignition on, but do not start the engine, wait 5 seconds. Switch ignition off, same procedure, then start engine.
On the E32 fuel pumps start running with ignition on till required fuel pressure is reached, then they switch off.
If that helps, check valve in tank.