Cat skin conditions

Anyone know anything about cats and possible causes for skin conditions? We've noticed one of ours has odd lumps on his skin. They're hard to see through his fur. They feel a bit like old scabs (which is a pleasant mental picture, I know) and he sometimes gets tiny bits of dark, hard material in his fur. We brush him regularly, he eats without problem and doesn't sleep any more than a normal cat. Obviously we can take him to the vet but if this is normal, I'd rather spare him the stress. Both our cats are indoor cats and the other one is fine so he hasn't caught something from another cat.

Any ideas, people?

I don't think it's fleas. Not seen any on him or when we brush him. Plus they don't scratch an unusual amout. Could be a food issue. I'll try him on a fish diet rather than meat.
Thanks.

Wayne K said:

Our cat had something similar and we switched to Purina rice and lamb and it cleared up.

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Ours like meat food (even if it does smell rank). If the fish stuff doesn't work, I'll try Purina.