A while back I installed a 32gb CF card in an h320 for a guy.
There was already an appropriate version of rockbox on the thing, and plenty of music, so I thought I'd be a nice guy and copy all his stuff to the CF card before installing it - with the CF card in a USB card reader.
I ran into similar problems, where it would chug along for a while and then suddenly just disappear. I ultimately got everything onto it, and figured that it was just a problem with my cheap crappy usb card reader, which had given me some trouble with something else.
And then a few weeks later he was listening to some music and it just stopped dead. When he reset the H320 it said it couldn't find the partition or something.
He gave it to me to test and the CF card was dead - it didn't show up in my (new) usb card reader, and plugging it into a pcmcia sleeve and inserting it into a laptop made it show up as a PATA controller with no drives.
So, my suggestion - do you have a usb card reader? Give it some stress tests in that. If it has issues in a card reader as well, maybe it's the CF card.
(fwiw, he wasn't using the kingston card, but he was using the topram card that identifies itself as a kingston card. Topram has agreed to replace his card.)