1969 280SL problem

What might be the cause of a lumpiness / bucking when driving slowly in first or second gear? These are the symptoms:

-slightly lumpy acceleration from 1000 to 2000 RPMs
-bucks when maintaining speed 1000 to 2000 RPM on a slight incline (1st/2nd gear)
-downshift "burbles" from 2000 to 1000 RPM
-idle no problem
-higher RPMs are fine
-throttle linkages are adjusted properly

Does this sound like an ignition problem? Fuel injection? Fuel pump? Vacuum advance?

Thanks in advance for your help,

Steve
1969 280SL 4 speed

[Aaron]

Have you looked at your points and checked the dwell angle?

Points and dwell are OK, but the fuel pump has been noisy for some time (screech). Could that be the cause? The 280SL is fuel injected.

Thanks,
Steve