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Wordpress site that times out and recovers randomly.
I have a VPS running Ubuntu 24.04.01. Using the Runcloud stack and Nginx. It hosts 3 wordpress sites. The main WP site works well, except for occasional times where the site will simply stop responding and will get a "bad gateway" message. My BetterStack monitoring tells me "Timeout (no headers received)" during these outages. Normally, the site will come back without any intervention from me in as few as 2-3 minutes, or sometimes as long as 20. These outages will seemingly occur a couple of times a day for a week, and then there will be no outages in a month or two. Can't seem to figure out what's going on.
During these outages, the server itself is still up, and the other 2 WP sites on the server still serve up fine. So it's something with this particular WP site.
The site was redesigned about a year ago. The theme is a brand new theme, but some portion of the plugins were present on the old version of the site as well. There are 2-3 plugins that put up a flag in my mind because of their developer (perceived to be low quality by me). But nothing else is immediately apparent.
I turned on WP debug logging on January 3. Just checked that file and it is about 25 GB now, lol. 101 million lines in the log :-) There are what I'd consider standard PHP deprecation messages all over the place, but I don't immediately see any smoking gun.
How would you start troubleshooting such an issue?

Comments
Definitely review all the plugins you have installed and look for old ones that haven’t been updated in a long time. If no future updates coming out, try find a replacement.
What does the php access and error logs say for the timestamp that the site experiences the issues?
Does your site run normal wp cron or server cron? Change to server cron if it’s not using that already.
Bad gateway can be a myriad of different things though, hard to properly point you somewhere without access to the logs. :P