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