I have the same problem, i have the stock ROM, GRK39F
My phone is not rooted but it's bootloader are unlocked and i have Amon RA 2.2.1 recovery.
i first encountered this issue when i tried to update app that installed on my SD card and each time i tried to update it the phone were automatically rebooted itself.
when i moved the exact app to phone internal memory the problem was solved and i was able to update it.
but when i try to move apps to SD card via the android settings
(Settings -> Applications -> Manage applications -> On SD card)
the phone reboot itself...
I will appreciate any assistance because my memory is very low and i can't install any apps