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rtrahms
Before I checked this, I did check current limiting Vref on the A4988s, and they all read 0.8v - which (x2.5) equates to a 2A current limit on each. Seems okay, but let me know if you disagree.
2A is WAY too much for those little plug-in drivers, because they have nothing like enough PCB area to remove the heat from the chip. The chips will be going into thermal shutdown, resulting in the erratic movement you observe. Try 1A.