Ridleusmc, this is just my 2 cents worth but I would try thinning with Tamiya thinner at about 50% first off. I know that Tamiya paint is water based but Tamiya thinner definitely gets it flowing smoother and spraying better. It costs more than water, I know, but believe me, it will prevent a lot of your Tamiya painting problems before they even happen. You say the problem is just with this colour, this would seem to rule out your airbrush as the problem. You could also try straining the paint through a stocking but really the lumps shouldn't be there in the first place. Maybe they made a bad batch of this colour? You could try getting a jar from another hobby store (hopefully from another batch). Sorry if this doesn't help, just thought I would throw some ideas at you.
Cheers...Snowy.