The transit and the flat battery

The old Mk1 transit has got some sort of electrical fault.After a run it seems as though every so often the battery is flat.I've had the starter off,the brushes are OK and the commutaors fine,I've checked all the leads,the earth strap,even rigged up temporary ones with jump leads and changed the battery.
All I can think of to try is the starter solenoid.The trouble is,if it's left for ten minutes it will then sometimes turn over and fire OK anyway and thats about the time it would take to fit another solenoid,and,it can go a week or more without doing it anyway.
Do you think my bum looks big in this?