Assuming everything is cool with your gain structure, and therefore what you are seeing is accurate, some MCL from CAIG is a good fader cleaner. And you can just spray it in there with the tube that comes with it. Make sure you get it past the little dust flaps.
ps- about as often as not, on mixers 10 years old or so, the faders actually need replacing, cleaning doesn't help much. So don't be too bummed if it doesn't work.
Thanks easy chair, the board has been well used but also well cared for so I'll try a little cleaning first. There used to be a product called tune-o-wash for tv's but the harmful propellants have since been banned (all the cool stuff that used to work also apparently kills us ! )