Have you checked the dashpot for oil?
If it get's dry then the piston doesn't lift smoothly and you get a flat spot (actually a 'jerkiness' in the uplift of the needle).
Sometimes with older SU's the bronze piston itself wears a little unevenly and tends to offer variable resistance (the oil dampening doesn't always act at the same rate... but this is usually only noticeable with instrumentation)
Check the dashpot and top it up with engine oil...some folks used to swear by brake fluid in the winter to aid cold running... but it should strictly be engine oil)
Ian