Finally replaced the old Celeron 1.7--256 Mb box with a new one.  It's an Intel quad core on a TH558HD mobo running XP SP3.  This with a Microproto DSLS though the parallel port, that up til now has never failed me.  For no obvious reason at random the feed [whether running Gcode OR jog] will jump to tick-tick-tick which works out to something like millimeters-per-fortnight.  I've even gone so far as to wipe the drive, reinstall XP, reinstall Mach3, etc.  It still does it.  The mobo has O.N.E. and a bunch of other newfangled stuff I don't understand yet...for now I'm back to the old Celeron box for the current job, but it is getting long in the tooth to say the least...anyone have suggestions on where I should start?  Lurch

Have you run the driver test?
Does it have onboard graphics?
Have you tried the optimisation steps detailed on the downloads page?
Hood