Another thing to look at. I had this problem on a 325. Since your battery is in the back of the car, the engine/computers/etc all relay on the chassis as the ground. Start at the back of the car and find out where the main ground wire from battery connects to chassis. Clean/tighten everything from the battery out. Then go to engine compartment and find out where the main(big) ground cable connects to the engine and then the chassis. Make sure all of those connections are clean and tight. This sounds like a bad connection issue to me, not defective equipment.

__________________
Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600