OK.. and is the motor reversing as well as stepping? (checking the DIR signal is changing as well)

What I'd do is check out the other channels Z and A (exercise A by telling Mach3 the A channel is the X channel i.e. use pins 8/9 instead of 2/3 in the configuration tab)

If they are all working its likely to be a solder bridge or dry joint on the X channel... easy enough to trace it through from pin 2 on the DB25 connector through the HC541 (using the datasheet to determine pinouts) to the 'stepX' connector.