Dear rotccapt,
Could be a couple of things.
1) seems we can eliminate the power supply as you tried several (smart move)
2) there are a couple of FAQs on my website that may give some guidance: No motor movement and Checking Voltages.
3) Touch the two voltage regulators (U4 and U5) during a 'failure' - are they getting warm/hot? (they should NOT)
4) confirming you do have a substantial heatsink for the driver chips AND a fan blowing across it?
5) Has your step/accel rate changed (potentially stepping to fast)?
6) do the axis motors or axis bind? can you manually move the axis freely by hand throughout their range?
7) do you have anything connected to the 24V fan connector? (TB 6)
a) are you taking ANY other voltages off the board (e.g. using the +5V to drive an opto board)
8) triple-check the stepper wiring:
a) steppers are wired correctly to the board
b) all joints, connections and screw terminals are secure
9) do you have any limit switches wired into TB4? (this on is a reach) - perhaps some spurious noise is causing an issue?
10) is the board clean? that is, nothing got on/in the board (dirt, dust, metal scraps, who-knows-what)
Regards, BrianV, HobbyCNC