ditto what Daniel said... but I have had the same problem before with one or two specific bottles of Tamiya paint. (not the specific color, other bottles of it worked fine) the paint lumped in front of the brush and it had gaps as I was brushing it etc... I chalked it up to an old bottle the LHS didn't rotate out...
I have had other bottles do similar but not as bad, I just put Tamiya thinner right into the bottle until it has like a watery "chili con queso" texture, or maybe a watery ranch dip if you haven't had the pleasure of dipping your tortilla chip in an authentic bowl of that lovely concoction..
Then it is ready to be taken from the bottle and thinned for painting... I would venture to say thinning would help a lot.... I use Tamiya thinner specifically with Tamiya paints.. it has a retarder which slows drying time and allows the paint more time to settle over the surface for a smooth finish, some people say it's expensive, but the 750ml bottle (the big one) lasts forever and is only a dollar or two more than the 30ml one!!!
and once again like Daniel mentioned... sometimes for specific colors especially... several coats goes a LONG way to a nice finish! (silver, white and yellow, and some gloss colors)
Good luck!!