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Paul,
H0ughy's cable theory is good.
Is there any correlation between the declination and tracking error? The closer to the pole, the worse the tracking?
With that theory, NGC3372 should be worst and the Messiers best. The RA balance or a cable pulling tight might be part of it which could mean if NGC5139 is bad, anything else being shot at a similar alt/az should be too.
On another couple of groups, the discussion seems to agree that as long as your polar align is good (not necessarily perfect), for high dec targets you can guide on just about any part of the sky within about 30 deg of there. With more available pixel movement the guider can be more stable.
Andrew
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