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A little bit of (imperceptible) bend in the lead screw or misalignment can definitely lead to binding (at least on LowRider, which is what I’m familiar with). As you observed, your fingers will give you more feedback operating it by hand than any other way.
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That looks pretty dry. Have you greased everything? I use white grease (lithium), but a dry graphite spray might be less messy.
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Lowering your Z speeds could give you that extra bit or torque you need.
#2 and #3 are workarounds for #1.