Mike, I have a 77 450SL also and just went through the same problem. About drove me nuts as it was intermitten and evry time I took it to the dealer, it would work fine. I went to pick up the car for the 3rd time from them when low and behold THEY couldn't get it to turn on either!!!!! They quickly determined that it was in fact the nuetral safety switch. Replaced it in about 30 minutes and haven't had any problems since!!!!!
Good luck!
Mike Tracy