I can understand your dilemma Geo, I've been a Humbrol user since I was knee high to a grass hopper but now as my supply depletes I'm converting to Model Master for spray work and Vallejo for most of the hand painting. I've also a supply of White Ensign enamel paints which are true to colour for WWII German, British or Japanese builds, these I get through the internet.
As for the problem you are having, try a bit of white spirit with more stirring and shaking and then maybe if you can, strain and rub it through a fine sieve, this may help to break up some of the lumps which I would say is the pigment gone real hard over time.