my ipod classic will not play podcasts

I have a 160GB iPOD classic and have been syncing successfully music, podcasts, photos and videos for many years. Originally formatted on my PC, for the last year I have only been syncing on my MAC but I haven't re-formatted the iPOD and it didn't seem to mind. A couple of weeks ago the iPOD decided it would not play any video podcasts (radio podcasts are fine). I select it to play, and after a second or two it goes back to the menu. I have completely cleared the iPOD and restored to factory settings. After re-syncing it still does not work. Movies (typically MP4 conversions from DVD and the web) all play fine. I have done this on the MAC (factory reset, re-sync) and also on the PC (factory re-set, re-sync). Makes no difference. Today I deselected all podcasts, synced, then just selected a new video podcast (a TED video as it happens) but still no luck. Does anyone have any suggestions?