Intermittent 500 errors to multisite login

I have a multisite, sub-directoty install that has been running fine for about 6 months. Occasionally a 500 error screen will appear, perhaps once or twice a month. After some time the error no longer appears and the login screen comes back. Although I can’t see the login screen, the public blog is live.

My guess at what the problem could be:
At install, the hosting script allows you to install to a subfolder. This presented problems for domain mapping, so I then moved the install to /root.

At the time, I changed the database pathing manually with a text search and replace, and carefully changed all db entries to “/” instead of “/subfolder”

I added <IfModule mod_rewrite.c> tags around the original htaccess install code, which didn’t seem to do anything.

The last error log entry was 6 days ago for an out of memory error. There have been no recent updates or upgrades and I was able to login this week and yesterday.

Any suggestions how to proceed?