Around yr2000, researchers at Purdue University, US, noticed that some radioactive sources - which should decay completely randomly - sometimes show peaks and troughs (i.e. cyclic variations) in the averaged radiation they produce over time. Subsequent tests carried out the Brookhaven National Laboratory and the Federal Physical and Technical Institute in Germany confirmed the phenomenon - which could not be explained by any conventional theory.
In 2006 a clue was provided when a fluctuation was logged during a large solar flare episode. Some theorists now suggest that the fluctuations might in some way caused by neutrino emissions from the sun (which vary according to sun cycles).
Although the neutrino theory holds up as far as linking the observations with sun activity - there is no known physical mechanism by which they can affect radioactive decay.
“What we're suggesting is that something that doesn't really interact with anything is changing something that can't be changed.”- say researchers.
See : The strange case of solar flares and radioactive elements Stanford Report, August 23, 2010.
Also see Radioactive decay
Also see Neutrino mass