Hi Alan, only just seen your post. i have had this problem before, get the engine up to operating temp with a test drive, then scope the camshaft sensor, all the ones i have done drop out after about 15 mins. this engine only needs cam signal to start if it drops out when the engine is running it will keep running

It's quite a common fault on sprinters they have modified the part about 3 times from memory ( touch wood i never see them back after and they are on our fleet )

i normally test the wiring through then replace both parts because all the one's i work on must be good vans they have been to the moon and back several times.

HTH
Kev G