Hi cormie,
erasing all the Rockbox files and clearing the setting should be sufficient for "clean reinstallation". If you already did all this and you still have the same problems your hardware is almost certainly faulty. Does the player crash when you run the original firmware too?
Wiping the whole harddisk is risky for non-expert users as you can accidentally erase hdd in your computer. It could be also complicated to recreate the partition table. OK, it was my fault to mention the option of totally erasing the hard drive

so I wrote a short tutorial.
If you want to run the risk, here is a tutorial found by google:
http://www.linuxquestions.org/linux/answers/Hardware/Clean_Hard_Drive_zero_fillBelow I describe the basic steps. You have to find details by yourself by googling or reading Linux HOWTOs.
1. Create the Linux boot CD / floppy / flashdisk or whatever.
2. Boot the CD.
3. Connect the player.
4. Check your SCSI devices:
cat /proc/scsi/scsi
You should get something like:
debian:~# cat /proc/scsi/scsi
Attached devices:
Host: scsi2 Channel: 00 Id: 00 Lun: 00
Vendor: TOSHIBA Model: MK2004GAL Rev: JA02
Type: Direct-Access ANSI SCSI revision: 02
This means that the player's hard drive is connected on virtual SCSI controller number 2.
5. Check The partitions on the SCSI device. If you have also other than the player's SCSI device connected (listed by the previous command) use sdb, sdc etc. instead of sda:
fdisk -l /dev/sda
Other possibility (not available in every Linux distribution)
fdisk -l /dev/scsi/host2/bus0/target0/lun0/disc
The result should look like (here it is 20 GB drive of H120):
debian:~# fdisk -l /dev/sda
Disk /dev/sda: 20.0 GB, 20000268288 bytes
255 heads, 63 sectors/track, 2431 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 2431 19526976 c W95 FAT32 (LBA)
Make sure that you have identified the correct drive or you will erase a wrong drive!6. Backup your MBR and partition table:
debian:~# dd if=/dev/sda of=H120-MBR.dat bs=512 count=1
1+0 records in
1+0 records out
512 bytes transferred in 0.362247 seconds (1413 bytes/sec)
Move the backup file to some safe place (flash disk, internal PC disk etc.). You have to mount the destination disk in Linux like this:
mount /dev/hda1 /mnt/windows
Many Linux distributions do not support NTFS by default!
7. Wipe the HDD by overwriting it by zeros:
debian:~# dd if=/dev/zero of=/dev/sda bs=16k
8. Reboot to Windows
debian:~# shutdown -r now
9. Recreate the partition table and format primary FAT32 partition. You can use for example
SwissKnife to do this.