What causes lines in the middle of a 3d print? Solve under extrusion problems mid print, halfway up!

At my wits end with this thing. I keep getting under extrusion mid prints causing horribly gapped layers and incredibly brittle parts. I finally ripped the hot end completely apart and I'm

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What causes lines in the middle of a 3d print? Solve under extrusion problems mid print, halfway up!

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