Final result: MB dealer says the no start was caused by a split vacuum line for the idle control valve, and the stalling with the A/C on at idle was caused by the main ground being loose. That's it. Interesting how the symptoms of one thing; bad OVP & MAS control unit/fuel pump relay, match that of another; split vacuum line. I have renewed trust in the dealer now, but I'll still continue to rely on this excellent forum for my DIY efforts. Thanks for all the suggestions.