Sure doesn't look like you're overwatering. If you've had crazy leaps of high and low temperatures, that could be causing it, but you would think it would affect all the plants.
The double edged sword of growing heirlooms is that they are not as disease resistant as hybrid varieties. It could be a soil-borne disease of some sort, and that would explain the randomness.
I'd give extra water in the mornings on days predicted to be hot to those affected plants and see if that helps. Tomatoes really are thirsty, hungry plants when they're first growing. You can't overwater, they'll just grow faster once the weather stays warm.
If extra water doesn't help things right away, a douse of a mild Hydrogen peroxide solution might be a next try. It can knock down things like fusarium if caught in the early stages. About a 1% solution or 4oz. peroxide per gallon of water.
