Thanks. I tried those values and it didn't have any noticeable effect. As an unscientific observation, there doesn't seem to be any stalling of the motors. It really appears that the commands are somehow being ignored.
I did a simple test - moving the Z axis up and down repeatedly to different values. It ran fine, never skipping a beat.
G21
G90
G28 xZ
G1 Z10
G1 Z5
G1 Z15
G1 Z3
G1 Z0
When I added a single X axis move to the script, it would skip or miss a Z axis move randomly.
G21
G90
G28 xZ
G1 Z10
G1 Z5
G1 Z15
G1 X25
G1 Z3
G1 Z0
Additionally there may be a long pause between steps.
I'm stumped.