HELP - Intermittent Wordpress Error 500

[ITGURU]

This is becoming a pain!

yesterday morning our website (internally hosted) was coming up with an error 500. Checked the server, nothing in event logs.

After an hour or so, website returned and was OK all day.

Come in this morning, users reporting same issue - website unavailable, error 500 - intermittently available.

Have tried rebooting the server, restarting IIS, re-adding the fastcgi config to PHP, and re-creating the application pool.

 

It's so intermittent, as was OK between 10am and just now - when I get a report from monitoring software that the website is down again with the error 500.

 

What's next to try?!

 

Server has been running happily for a few years, no changes, no recent updates, first time it has happened. No disk errors, CPU usage low (5%), 55% ram usage.