how can i fix my renault master 2004 diesel lm35 dci 100 lwb 2463cc?

after driving vechicle for about 20 mins, the stop light and oil pressure light both come on. if you turn off engine,and restart no warning lights if you leave vechicle stationery, no warning lights, however if you start driving, after about 3 mins warning lights come on. (there is plenty of oil in engine)
there has been a hand held device connected to van, but comes up with no recorded faults. can any one help please.