If there's a multi wire plug going into the relay then check this for broken wires. My 106 diesel had a problem that I tracked back to a broken wire RIGHT at the base of the plug.
From memory, the glowstart relay on these is very similar, so perhaps that's worth a try?
Check condition of all wiring from glowplugs to relay, sounds like a broken or loose connection to me.
As Andre said a step by step guide of your problem and how intermittent it is will help us diagnose..
How many miles has your van done??
Adam..2003..1.9D..LX..Silver Crew Cab Van.....
If at first you don't succeed, destroy all evidence that you tried.