Eventually denied permission to write in home directory

Question

Hello,
My wife is using a Mid-2007, 2.4GHz Intel Core 2 Duo iMac (iMac7,1) with 6GB RAM and more than 1TB of free available HD space.  She is running Mountain Lion (10.8.3) and has administrator privileges.
After she has been using the computer for a while (hours or days; it varies), the system eventually denies her writing privileges to folders in her own Home directory. 
I'll give two common examples:
She'll download a file in Safari (or iTunes), and everything appears to proceed normally until the download "finishes" at which point she receives an error message that says she doesn't have permission to write to the save location.  The download remains in her browser's download list, but the file itself "disappears".  Changing target directories (e.g. from Downloads to Desktop) makes no difference.
TextEdit will automatically reopen upon log in, and display one or more SavedState "Untitled" documents with text in them (she uses it as a scratch pad). When she attempts to add text to one of these documents, e.g. "Untitled 8," an error appears that says she does not have permission to make changes to the document and is prompted to make a Duplicate, so she does so.  She can successfully enter text in "Untitled 8 (copy)" but when it comes time to save the file, another error is returned that claims she does not have permission to write to [whatever location].
Only two things seem to correct this situation, neither of which are permanent. That is, it eventually happens again.
1. She logs out and logs back in. Things will behave normally for a while.  This is a pain, of course, because she typically discovers the problem when she is in the middle on working on one or more things, and the login process is fairly slow (as are most things on this iMac since upgrading to Mountain Lion -- though better since restricting the activity of Sophos Antivirus).
2. Repair the permissions in her Home directory:  Get Info, click the lock & enter her password, change the Privilege of Me from Read & Write to Read Only, then back to Read & Write, and finally click the gear so as to Apply to enclosed items.  Once this is finished, access is restored... until it happens again.  Funny thing about this particular procedure is that her permissions appear to be set normally at the outset, ie., Read & Write.
Another odd thing about the download problem is that the system does not seem to have a problem writing to whatever temporary directory it uses during the downloading process. The error only comes once the file is complete.
There does not seem to be any precipitating event that leads to this change in her permissions/access privileges.
Repairing permissions using Disk Utility does not resolve the problem.
Running other kinds of repairs (repair disk when booted in Recovery Mode, running Drive Genius 3 from a separate partition or from the optical media, TechTool Pro, etc.) do not resolve the problem.
When I log in to this same computer using my account (also with admin privileges), I have yet to encounter this eventual-permission-denial problem.
Any thoughts or recommendations are welcome, obviously.  This is driving us batty!

Answer 1

Hmmm, sounded like a RAM problem until you mentioned your account doesn't do it.
One way to test is to Safe Boot from the HD, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, Test for problem in Safe Mode...
PS. Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive
Reboot, test again.
If it only does it in Regular Boot, then it could be some hardware problem like Video card, (Quartz is turned off in Safe Mode), or Airport, or some USB or Firewire device, or 3rd party add-on, Check System Preferences>Accounts (Users & Groups in later OSX versions)>Login Items window to see if it or something relevant is listed.
Check the System Preferences>Other Row, for 3rd party Pref Panes.
Also look in these if they exist, some are invisible...
/private/var/run/StartupItems
/Library/StartupItems
/System/Library/StartupItems
/System/Library/LaunchDaemons
/Library/LaunchDaemons