Boot drive (SSD) going read-only randomly

Boot drive (SSD) going read-only randomly

I've got a mid-2010 MBP running OS X 10.8.4. I've got a Sandisk SSD as my
boot drive in my Super Bay and a 1.5TB hard disk in my HD bay. (I realize
I could get better SSD performance if I swapped those, but the HD is too
tall.)
When I boot my computer, everything seems fine. But after varying amounts
of usage time, the SSD goes unwritable until I reboot. It's extremely
weird. I can't touch to create new files anywhere (I get an error message
like "touch: test.txt: Invalid argument"), I can't save anything, and
applications like Mail, which depend on disk access start crashing without
error messages. The Console and logs don't seem to be revealing anything
unusual. Disk Utility doesn't start and diskutil hangs. If I manage to run
out my main memory, I get the "Boot disk is full, kill an application"
message. But even more weirdly, for apps that don't need to save anything,
the computer seems to work fine. Rebooting restores everything to normal
temporarily.
I'm not sure if it's related, but I also have these sporadic periods of
10-20 seconds of spinning rainbow ball, where kernel_task is my highest
CPU user and seems to block ALL other activity. Disk transfers, network
transfers, and all else seem to come to a temporary halt until whatever it
is its doing finishes. I can usually still Cmd+Tab to Activity Monitor
during these seizures.
I brought it to the Apple Store today, and their diagnostics couldn't find
anything unsually. They successfully repaired permissions in Safe Mode,
but the problem still exists. The tech there said that it seems as though
the kernel can continue writing to disk in this state, but nothing in user
mode.
Anybody have any idea whatsoever what might be causing this? My only other
option will be to reinstall the OS, but I'm trying to make that my last
resort.