Vintage Cars: 450SL sputters and jerks, vacuum leaks, premium gasoline

Question
I own a 76 450SL that I drive periodically.  I had the transmission rebuilt over the winter and did not drive it for about 6 months.  It will run good for a few minutes and then it will jerk, jerk , jerk getting worse and worse at higher speeds.  I cannot accelerate past about 50mph. It does this jerking at all speeds.  Eventually, when I come to a stop the engine dies.  It may restart but dies again upon acceleration. Last week I had the plugs and wires replaced along with the fuel filter.  It ran better for about 1 day (5 miles) and is now running worse than before. I have only used premium gasoline along with additives.  Not sure what to do next.  The engine has always had a high idle (I have owned it for about 15 months).  Not sure whether to have this towed to a Mercedes dealership for service, or return to my local mechanic.

Answer
if it sat for 6 months you very well may be looking simply at poor fuel, however these cars are very sensitive to vacuum leaks.  A leak anywhere can cause this issue.  You could also have clogged injectors
or gummed up fuel lines.  Letting a car sit can be hard on them.  If it were me Id start with looking at the fuel system ,and consider flushing out old gas.  I would then focus on the injectors. Of course make sure that the timing is correct. Any competant foriegn car mechanic should be able to help. I would avoid dealers, but thats me, I do all my own work.