Help please: Stuttering Stepper Motors in Jog Mode

Dear All,

I have recently converted my Denford Orac lathe from Parker CD20 to
Gecko 210 stepper drives. The lathe is driven from a laptop PC using
TurboCNC 4.01 software. The CD20's needed 24V pulses hence were
connected through a breakout board, but the Gecko's are now directly
wired to the parallel port (+5V to Common and the step and dir pins
to the step and dir terminals).

The new setup suffers from the most peculiar fault:
When I jog an axis along by pressing the cursor keys the motion of
the axis is erratic, jerky. It stutters like a misfiring engine or
jamming machine gun. The distance travelled has little to do with
how long you press the key.
However, when in MDI mode, and I enter G1 Z10 F50, it moves along in
a perfectly smooth manner. (Same goes for any other F from 5 to
1000.) The distance travelled will be exactly 10 mm, or whatever I
have asked for.
I pulled the old `scope out and had a look: In MDI mode the pulses
come perfectly harmonic, not missing a beat.
When using the jog cursor keys however the scope screen matches the
stuttering sound and jerky travel: The rectangular wave is totally
irregular. Amplitude is nearly 5V in both cases.

This is one for the experts: How can the step pulses on the parallel
port be so irregular when generated via cursor keys, and perfectly
OK when triggered by software alone?

To be sure, the G210 is OK since it also works perfectly well with a
TTL signal from a generator, and the laptop is also OK because I
have tried a different one (different make altogether) and it gives
100% the same results.

It's got to be interaction between the drives and the PC, but how
and what? Why did I never get anything like this with the CD20
drives?

Any suggestions appreciated.

Best regards,
Thomas.