I reloaded mine (480GB Crucial chip) to 350GB and used it for a few days with no problems at all.
I then took it up to around 375GB and on reboot got the following error message immediately after the Rockbox menu appeared:
SWI at 0000002C (0)
The iPod started getting hot at the same time.
I powered down, connected it to the computer and restarted. It went into USB disc mode no problem. I rebooted while connected to the computer and then ejected the iPod. It restarted without any problem and has been working fine ever since.
I will try adding some more files tomorrow and check its behaviour again.