I have similar problems...with more symptoms.
Solenoid has not yet been brought up in my posts.

Question: When you turn your key all the way over, do you hear a 'click'? Is there any symptoms PRIOR to this happening?
I know yours is not draining your battery, but mine sometimes does, but most the time does exactly what you say...Starts eventually.

Here is a link to my posts:
300TDT won't start!

My God, Another problem

I will be following your posts as well. Hopefully we can both get our Diesels exorcisized!!

__________________
Current: '91 300TE 4MATIC 317k and climbing...
Former:
'81 300TD Wagon 168K "Tank"
'83 240D 216K 4spd manual "Da Bear" (aka best car ever)

"Never sweat the petty things...
and never pet the sweaty things."