7-segment gas discharge display - segments are not lit up completely
OK, after being pointed into the direction of gas-discharge displays (e.g. nixie tubes), I found out that this is a case of 'cathode poisoning'. The images on the linked page match with what could be seen on my display (dark regions on the segments).

The remedy is to drive the segments with a current 2 up to 10 times higher than what is normally used. The DS8880 drivers are configured to about 0.4 mA segment current, which matches with the data sheets of the display.

So I connected a 4.7kohm/2W resistor in parallel to the display driver (between the segment connector and ground) to increase the current. I measured about 0.5 mA through it, so when the segment was already lit up it got double the current (for non-lit segments I used a 3.3k resistor). I let the resistor stay there for each segment about 2 to 3 hours, until the segment was lit up evenly. Interestingly in the course of this the current through the resistor increased (up to 0.7mA), so the resistance of the segments was going down.

The result can be seen here:



I will now just need to see whether this will happen again soon or not.