I'd bet you're going to find it's a connection somewhere - most likely 
with reference to the battery: I bought my Vanagon new & drove it over 
10k miles before it did exactly as yours (at the end of a 25 mile dirt 
road in NoWhere, Wyoming) - turned out the battery ground cable was 
snugged down but never tightened.  The worst part was re-programming the 
radio ;).