Ubuntu Guest IO Hangs

[tpearsall]

Host: CentOS 2.6.18-308.16.1.el5PAE #1 SMP (SME Server)
Vbox: VirtualBox-4.2-4.2.0_80737_el5-1.i386, Bridged LAN
Guest: Ubuntu 12.04.1 64-bit with Guest Extensions
Config: Guest runs MythTV Backend services accessing video files on the host mounted using vboxfs

I have been running the MythTV Backend server v.24 for a couple years on Vbox 4.1.8 on a Ubuntu 10.04 LTS 32-bit guest. When Ubuntu 12.04.1 LTS came out I created a new 64-bit VM to replace it and upgraded to MythTV Backend .25. (Yeah, I know I that I changed too many things as once...). Since the change after anywhere from 10 minutes to a few hours of running any disk activity to the vboxsf file system hangs. For example "df -h" or "ls /mnt/mythvideos" will hang. The guest is still running and you can navigate the OS as long as you don't try the mounted volumes. Hoping it was a vboxfs issue, I tried using cifs mounts to the host instead, but get the same hang. I upgraded to Vbox 4.2 and the problem continues.

fstab entries:
videos /mnt/mythvideos vboxsf gid=117,uid=110
recordings /mnt/mythrecordings vboxsf gid=117,uid=110
music /mnt/mythmusic vboxsf gid=117,uid=110
photos /mnt/mythphotos vboxsf gid=117,uid=110

showvminfo: http://pastebin.com/GWQecDaj

I haven't found anything suspicious in the host logs, but there is a crash on the guest.
<snip>
[ 8783.052320] VbglR0HGCMInternalCall: vbglR0HGCMInternalDoCall failed. rc=-2
[ 8783.111869] VBoxGuestCommonIOCtl: HGCM_CALL: 64 Failed. rc=-2.
[ 8783.256511] BUG: unable to handle kernel paging request at 00000b5700000057
[ 8783.256538] IP: [<ffffffffa00468c8>] vbglPhysHeapExcludeBlock+0x18/0x60 [vboxguest]
[ 8783.256597] PGD 0
[ 8783.256602] Oops: 0002 [#1] SMP
[ 8783.256607] CPU 1
[ 8783.256609] Modules linked in: vboxvideo(O) drm vesafb ppdev vboxsf(O) psmouse serio_raw parport_pc i2c_piix4 vboxguest(O) mac_hid lp parport e1000
</snip>
Full syslog details: http://pastebin.com/VV8KELMw

At this point I can't tell if the issue is Vbox, Ubuntu 12.04.1 or MythTV .25

Any direction on troubleshooting would be a huge help. Thanks!