Your supply is probably OK for driving the eggbot. The problem seems to be your current potmeter setting.
With the pot at maximum the current to the motor is too high – the driver chip protection circuitry kicks in – causing the jittering. Try setting the pot to 11 o’clock position (full travel from approximately 8 to 4). On my ver13 boards this results in a current around 570mA which works nice.
(Please be aware that the silkscreen print on the board with indicates counterclockwise rotation for increase in current is incorrect.)
Good luck. Please feedback on results.