Even Dalrymple (1984, p. 88) recognizes such irregularities: Under certain environmental conditions, the decay characteristics of 14C, 60Co, and 137Ce, all of which decay by beta emission, do deviate slightly from the ideal random distribution predicted by current theory ... , but changes in the decay constants have not been detected.  Dalrymple cites the references Anderson, J. L., 1972, Non-Poisson distributions observed during counting of certain carbon-14-labeled organic (sub) monolayers, Phys. Chem. J. 76: 3603-3612 and Anderson, J.L.and G.W. Spangler, 1973, Serial statistics: Is radioactive decay random? Phys. Chem. J. 77: 3114 - 3121.
Dalrymple claims no changes in the decay constants have been detected, but he admits puzzling irregularities in decay.