[fw-db] MySQL PDO Write Error

I got the following exception in the project located on windows server:

exception 'Zend_Db_Statement_Exception' with message 'SQLSTATE[HY000]:
General error: 1 Can't create/write to file
'C:\WINDOWS\TEMP\#sql_c48_0.MYI' (Errcode: 13)' in
C:\...\Zend\Db\Statement\Pdo.php:227

I know that the error means "Permission denied" (OS error code 13:
Permission denied) but it happens INTERMITTENTLY! The TEMP directory
has write permission for the web server user.

How is this possible? Sometimes the same script has permission for
this directory and sometimes it doesn't?

I would really appreciate any help on the issue. Why it might occur
and how to fix it?