One of the things that sucks the most with mixers is shitty faders. Now I'm no expert, but in my experience, it is usually due to a build up of crap (dust) on the tracks the fader runs on. If you have a reasonably new model, you can just buy replacement parts.
If your like me, and have a discontinued model, trying to find a replacement is a pain in the ass.
I choose to save some money and clean them myself.
It may be that not all faders are the same, but they 'probably' work on the same lines (as I have said b4 I'm no tech).
This is the process...

First, you need to desolder the fader from the board, using a solder sucker.



Next, remove the lugs that hold the fader case to the board. This one has 6 (the folded over bits, just use small pliers)



Gently pull the board away from the case...



Pull out the centre sliding peice that the knob fits on to.



This shot shows the gunk that has bulit up on the connections...



The track the connections run along...



Using an old toothbrush, clean along the strips. There are probably better ways of doing this, like isoprpyl aclohol etc, but I don't have any handy to me. This seems to work ok anyway.



Next, clean the connections of the slider part, just be careful not to damage them as some I have seen are quite delicate. These may also need to be 'bent out' a little to make a stronger contact with the track.



Once done, put it back together and fold the lugs back over.



Your then ready to resolder and re assemble.




Hopefully this will fix your dodgy mixer. If not don't blame me, it's probably a more serious issue. if so and you need a new fader, it won't hurt trying this procedure first anyway. It only took me about 30mins start to finish.

One thing to be careful of is the way the board fits to the chasis. In this case one side had a larger cutout in the board which meant it could only go one way. however when doing this with my pitch slider on the 1200, the board can go either way, and the clider track is slightly different in that one side of centre has 2 tracks, and the other has 3. I have no idea of the importance of this difference (maybe someone else can chip in here), but best to keep it the same.