1971 280sl stalling when hot

[aaown]

I am at my wits end. I have a 1971 280sl that runs great when you first start it and drive down the road. Take it on the road and run it for 5 to 7 miles at 60mph and it will start to stumble, lose power and eventually stall. Pull over, let it sit for a minute and it will start and run ok for a few minutes and do the same thing all over again. It is running rich as the plugs seem to have some black residue on them, but not wet. I have replaced all filters (air, gas, injection pump), standard ignition items (plugs, wires, cap, rotor, condenser, coil), no help.
Any idea's out there? Please let me know if you have any idea's.

Thanks,

P.S. This car has about 50k on it. It is a 6 cyl with a 4 speed. It has been in storage for about 10 years, being started only occassionally with treated gas.