I may have just stumbled upon the cause of your woes. My 406 was suffering from similar problems. Whilst cleaning earlier to day, I noticed that the plastic impeller cover was warped into a U shape. I immersed the cover in some freshly boiled water to soften the plastic, then bent it back into roughly the correct shape. (Slightly over doing it so as it would sit tight when back in position.) and then run it under a cold tap to harden the plastic again. Instant improvement. I hope that this is helpfully to you.
Thanks! I'll take a look at that when I put the purgen back in the filter. Do you know what causes it?