So I'm leaning towards crank sensor but I'm far from certain at this point. I went out a couple nights ago and at about 11pm hopped in the xfire (2006 limited with the tvt265 kit). Turn the key to "on" and let it prime the fuel pump for a min (I only drive the car about once a week), then go ahead and give it a solid flick and it cranks right up. As soon as it's running, however, it stalls, and all power is gone from the dash. Tried to crank it again to no avail (dead, zero power to anything from what I could tell, no starter click or anything).

I turn my headlights on and after a fraction of a second they click on and it's like the old girl got water thrown in her face, all power came back across the dash and she cranked back up fine. Took her for a drive with no problems at all and haven't had a problem sense.

I've had random crank sensor problems on other vehicle so I know they can be beyond intermittent but I'm not familiar enough with the starting circuit on the Mercedes to know if this is something that could happen. If it throws a red flag to anyone out there please chime in!

edit: Once she cranked up fine there was no CEL or anything, don't have access to a scanner to read any hidden codes.