EUREKA! (I think.) What about this explanation: http://www.oznet.ksu.edu/hfrr/extensn/problems/Tomato,HardCore.htm Apparently a stress-induced condition, and more comon in some varieties (like ‘Jet Star’) than others. Read this from Kansas State extension. In tomatoes, stress is usually from uneven temperature and/or uneven soil moisture… I agree a soil test is a good idea. I wonder if you grow the same variety every year?