Stitch spacing too small
This is usually due to a lack of overexcitation drive. Check that the 40 volt fuse is OK. If the fuse is OK try removing the fuse and check that the red LED lights up, if not there is a fault in the power supply. If the fuse blows again when replaced there will be a fault on one of the driver boards. If the fuse only blows when the machine is operated look for short circuits around the clutch-brake assemblies.
Unless a failure has been caused by a short circuit it is most likely that the top driver board (roller drive) is faulty. This is because this board is more stressed than the head driver. The most likely failure is an emitter to collector short-circuit on a driver transistor. This can be checked with an ohm-meter (disconnect the power and the 16 pin connector to the panel before checking).

Stitches get more spread out as the machine is operated and close up again after a rest - or variation of last stitch position
Check that the 40 volt supply is OK (as described above). If so, the most likely cause is the roller clutch-brake unit. Check that the running gap is correct and that that no grease has got on the linings. Also check the drive pulley and belts for signs of slippage. If the gap is too small the clutch brake assembly will get overheated.

Erratic stitch spacings - usually seen as variation in the position of the last of a long row of stitches can be due to the clutch-brakes on the rollers. I have seen a machine with this problem with a new clutch-brake assembly fitted. After much grief the fault was cured by swapping the head and roller clutch/brake units. Usually the stitch head clutch-brake will be in good condition and can be swapped with the rollers. The head clutch is much less critical.

Machine puts second set of stitches in the middle of the box

This is caused by the box sensor re-triggering half way through the box. Can be caused by a heavy print on the box, or if the sensitivity of the sensor is not high enough. Can also sometimes be caused by a build up of dust on the sensor lenses.

Sometimes get a torn stitch in the middle of a box
Disable the rising table trigger arm and see if the fault disappears. If so – check that the switch contact is fitted with a suppressor.

First stitch position sometimes too short
I came across this problem only recently and it turned out to be the box sensor was seeing the box about 6mm before the box was caught in the nip rollers. All was fine if the box was pushed into the machine smartly, but if there was some hesitation the first stitch was short. Make sure the box is in the rollers before the sensor makes.