I would expect current mirror in this driver. So current flowing throught segments should be quite constant.
Check your power supply, add some decoupling capacitors, try to reduce segment current...
Thanks - I've done all 3. Even blown a few segments with too much current - part of that comes down to me not fully understanding the charts provided for Rset so I experimented a bit. And why is it that data sheets for these old 7segment segments cannot be found? Something provides a hit, but you get a PDF for a completely different model/vendor. Anyway - I made a few guesses but I was NEVER able to reliably find forward voltage to find the right entry.
Anyway - there's definitely a current difference as you enable/disable 7segment modules (this chip allows you to enable only some of them, so if you're only showing "12" on a 4 module display, you can turn off the first two. When I do that, "12" is brighter than when I have 3 or 4 digits on. Even when I do a test with all segments turned on, it seems some of the segments are slightly dimmer (always the same ones). I'll experiment with other Rset values and see what happens.
Thanks for a good reply - got me thinking and learning something new!