System/internal drive becomes read-only during use

While using my laptop, sometimes my system drive becomes read-only. I can no longer write files, create directories, or execute programs that require writing of files to launch or function. I have searched for others experiencing this problem but have so far found nothing. Restarting the computer seems to fix it for some time. This happens sometimes a couple times a day and sometimes not for weeks, and so far I haven't been able to figure out what might be causing it.

The hard drive is in good condition according to both diagnostics of the hardware and of the OS. Nonetheless I reinstalled the OS, but it has not helped.

I have a late 2011 MacBook Pro running El Capitan.