Was the Cali one of the players that the Cambridge team worked on?

If not, there might not be anyone here knowledgable enough to help you. Definitely search around for other forums to ask the same question. Not sure if there are any, though. Does Riovolution cover the flash memory players?

Anyhow, with zero knowledge of the flash players, my first guess would be problems with the contact points between the SD card and the player. Either crud on the contacts themselves, or broken/intermittent circuit traces or solder joints leading to the contacts. Also, a bad/flaky memory card might be the cause.