Thanks, but this is happening when ships are not performing a burn, i.e. at 0 thrust, with RCS & SAS off - so CoT & CoM are irrelevant in this case. I just put a probe core + 2 symetrical batteries up to double check, no RCS, no engines & it still span. In this case alt + x followed by f5 - f9 fixed the spin, so I guess it must just be a trimming issue?