[Brabus3.6]

Yesterday I removed the distributor in the process of removing the valve cover to reset the valve clearances. I then saw that the rotor arm was badly burnt & the centre electrode of the distributor cap had burnt "singed" in half. I duly replaced these items only to find a temporary (few minutes) better running & then the same problem again. Tonight I took the car out for a spin & gave it a quick blast. On my way back home I was coming to a standstill expecting RPM to drop to 500 & so engaging my right foot when I noticed it idling happily in drive at 650 RPM. So I took it for a short drive and it was fine. I left it idling for 5 minutes outside my house & when I returned it was idling in neutral at 550RPM, sure enough I dropped it into drive & it sank to 500RPM. It's like the car's teasing me!
An OVP question. Today I removed the OVP to find it had 89 stamped on it so it's the same one since the car's inception. With the car cold started I removed immediately the idle slowed, roughened and died. Does this mean it's okay or could there be a problem with it which shows up when warm?
Okay, I think I've just isolated the problem. I was driving in daylight today hence no need for lights and I didn't have my radio on (two amps, components & a sub, it's juicy on the current), the car was working well. Then while in traffic I put he radio on with a song that was bass heavy, drawing lots of current to drive the sub. Idle immediately dropped. So I put the radio off, blasted home, idle outside my house was fine. I put the lights & radio on, idle dropped to 500 again. Switched them off, revved it, let off the gas & idle increased to 600. So I believe this to be the alternator. I'm off skiing in France this morning but as soon as I get back I'm on it. I need to upgrade the alternator anyway since it's still the original 70amp unit. I'm thinking the 115/120A units should be sufficient but I'll grab a 143A or 150A if it's not too hard to source.