Leewolf, I have EXACTLY the same problem on my '79 450SL (77,000 miles). Starts excellent when cold. Starts fine after 30 mins, 1 hour, 2 hours, even 10 hours. But after about 4-6 hours, it becomes difficult. I crank it over 3-4 times for about 10 secs each before it starts and then it runs extremley rough for 5-10 seconds as I'm giving it some gas to keep from stalling. Most recently happened yesterday leaving a 4th of July party.
Problem; loss of pressure in the fuel system. Culprit; warped fuel distributor. Earlier model 107's used a cast iron distributor, later models an aluminum, (no, their not interchageable). Over time the cast iron model tended to warp, depressurizing the fuel system ever so slowly. After 10 hours or longer the cold start kicks in and makes it easier to start. Problem could also be leaky fuel injectors so you might want to start their. The fuel accumulator, also. Rebuilt fuel distributors run around $1400.00 and about half that used. Injectors around $30.00. Dealer quoted me $2951.94 for new distributor, accumulator and injectors. Needless to say I haven't had it done yet.
I read a post here, I believe, about a similar problem. The owner said to give it gas as you tun it off and then when you try to start it, turn the key only to the on position and then back to off 4-5 times, which is supposed to charge up the fuel system I believe.
Hope this helps.