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Backlash.
See how the correction pulses (red lines on the bottom) try to do the correction but noting happens for a while and then suddenly there is a correction. This is a clear indication of backlash.
1. Which mount?
2. Run the guiding assistant in PHD. It will measure the DEC backlash for you to confirm my guess.
By the way, the popular HEQ5/EQ6 almost always have some backlash in DEC that cannot be removed. The usual cure is to guide in one direction only and also to keep the mount off balance to keep the gears engaged.
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