From what I've read so far you haven't been systematic when troubleshooting.
1. While troubleshooting, make sure you're connected to AC power.
2. If you boot into original/legacy Iriver firmware (ie not Rockbox), does it behave as normal?
3. If you connect it to a computer, have you ran the utilities to check the disk integrity and fix errors? (search the forum here for which commands and switches to use). What results did you get?
4. To eliminate all uncertainty, format the disk using the built-in format function of the legacy Iriver firmware while disconnected from the PC (search the forum if you can't find the right submenu). Don't use the format function in Windows, it won't work.
If the disk is flaky, consider doing the CF mod.
If the disk is proven faultless, I have to leave it to the smarter guys here.
I'd put a dime on disk issues...