The cause is mainly the (tap-) water quality. If you have hard water (dH > 12 ) you can have this. Use a Brita (TM) filter which takes away Kat- Anions and have a Carbon fine filter. I agree with the pre-soak for C41. It can also eliminates a lot of problems. 3-5 minutes is sufficient. I am using a Jobo CPA-2 (with elevator) in 35mm and roll film development. Drum 2523, 270ml as minimum for 2x 135-36 or 2x 120 roll films. C41 dev. F position, Blix (or Bleach/Fix) P position for agitation. I have always very clean negatives. Kit: Amaloco C41 in the past. Actual: Rollei Digibase C41, in fact a small Fuji Hunt C41 set.