(Just a shot in the dark...) Are you missing a ground wire? Make sure you've got heavy connections between battery negatives, engine, frame and body.
Oh, lots of the time critical grounds from things like ECUs and sensors will all ground to one point, maybe on the engine. Bottom line, make sure they're all present and connected where they're supposed to be.
The Constitution needs to be re-read, not re-written!
If you can't handle Dr. Seuss, how will you handle real life?
Current oil burners: MB GLK250 BlueTEC, John Deere X758
New ride: MB GLS450 - most stately
Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510