The first thing that popped into my head while reading this is a sticky ignition switch possibly shorting a small amount of current. This might explain the intermittant nature of the problem. Do as the others suggested and do an amp test and if you get a reading, try wiggling the ignition switch... Just a thought...

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250