I have a 73 450SL as well. Check your vaccum lines. One to check in on the passanger side by the by the coolent overflow for the Radiator. What that does is retards the timing upon warm up. Sounds like something isn't quite right there. May connect a timing light to the car cold and then start it. You may see a change from cold to when it smooths out. That car timing is 5ATDC.
I have a problem with Exhaust Manifolds glowing when run at 3000 RPM's for 5 minutes, pull over at night and lift the hood and the tails of the exhaust Manifolds Glow. Does yours do that. If you would try that and let me know I would GREATLY Appreciate it. I have been chasing this one for 6 months. If you need further help in more detail, I may be able to help.