Have a vehicle in that's driving me nuts. It's an S420 w/ 149.000miles. replaced engine upper harness, crank position sensor, cam sensor, MAF unit, IAT sensor (in that order); still have trans. overload switch faulty code I have not taken care of yet. Wires and ends are original, caps and rotors are old (dual coil ignition system). CC/ISC actuator is original and marginally bad.

The Problem
Vehicle runs fine when cool, fine when warming up. As soon as it starts running hot (~105C), vehicle begins to lack power and will cut out (became worse shortly after I changed out the wiring harness). Scanner waveforms show charging circuit cut-out around 1/2 second before fuel pump or ignition circuit cuts out, but first time the charging circuit cuts out, the fuel pump and ignition stays running (at approx. 400-500 RPM). It will start right up, then die after ~5 seconds, then start and die almost immediately, until it sits for 10-30 minutes.

Vehicle has a HUGE sound system (w/ a back-up battery), although doesn't look like a cap or diode is hooked up in conjunction; also has a security system w/ remote start (aftermarket). Changed out ignition control unit, base control unit, CC/ISC control unit, and LH unit, and it does the same thing (cuts out at high temps). Any ideas? How about reason charging voltage cuts out before fuel pumps or ignition system quits firing?

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