I posted this on the Australian MB forum but in case some of you do not go there I thought I'd post it here too.
This is a part two of: http://www.mbcnsw.org.au/forum/viewtopic.php?t=209
See what you think...

Night time, I start the car, lights on, the fuel gauge sticking as usual, I tap it gently - all interior lights die! Dashboard, console... Everything! Exterior lights are OK, the CD is playing. So, I thought, the fuse No.5 had it.
I kept going (city lights were sufficient enough to see the instruments) and soon realised something was very DIFFERENT! The car took off like a rocket! It was so responsive, felt so powerfull, it leapt at a slightest touch of the pedal. The engine was quieter, the revs were lower yet - I drove for 45 minutes enjoing my "new" car.
The conclusion is obvious: my car never worked properly (since in my posession), it had power but at the expence of higher revs and fuel consumption.
I concluded the fault was electrical and not mechanical. That's why turning the Aircon on was severely affecting the performance, I thought. I am pretty sure that turning the fan (blower) on without the a/c it affected the revs, even the CD player...
The next day I wasn't well enough to drive till after dark when I had to drive to the Supermarket 'cause the fridge was empty.
After two minutes the lights came on, I didn't touch anything, and you guessed it - the performance dropped! But not to the level prior the lights dying, a bit worse than that... And more thirsty!
These are my observations: when the music is on I can hear the engine buzzing on the speakers, the console fader not effective at all, couple of times the music stopped mid song, it comes back on after a minute or so but not before there is a sudden "boom" as if a connection is momentarily made...
Further, it seems that the car performs better when cold, say the first 5 minutes, the temparature reaches 80 within that time, passes that mark only about 2 to 4 milimeters in a stop n'go traffic but goes back to 80 as soon as there's a mile of steady driving, even at only 60kmh.
The pressure gauge is firmly on 3 while driving, it does not quite drop to 2 when idling at the lights, all seems good.
So, the temparature and the pressure are the only consistent performers (the gauges never stick) but everithing else varies and behaves strangely.

Someone I know has mistakenly blead a portion of the trany fluid from his 1 y.o. Corolla (he wanted to change the oil) and reported that the car would crawl despite the engine sounding healthy until he topped up the trany fluid. 6 to 7 weeks ago my level was fine, should I check again, could that be part of my problem (I'm writing this in the small hours of the night, can't go checking now)?
I think an expert with a multimeter (like Mathew) could provide answers so Mathew I'll try to get in contact with you if you're still willing to help.
Or anyone else for that matter, if you live within 200 km from Newcastle I could come to you.
Thanx all for your patience with me.
P.S. If you go to my http://ZedMercedes.isCool.net you can se what's in my boot regarding audio.