As suggested above - I removed the central wheel and problem solved.
In fairness I do have a few "naughty" curves made from flexitrack that are pushing the envelope so I wasnt shocked - all my bogie stock and 4 wheelers have had no problems - most of the locos too...with exception of some of the more stubborn members of my A class fleet..between derailments, oil leaks and gears slipping and motors giving trouble...i know we want prototypical..but Jaysis!
I might do as Old Blarney has suggested and give each wagon a good check up and a bit of lubrication if necessary.