Help! Site slow and not loading, 500 & 503 errors

I've got an old 5.6 site that's loading painfully slowly, throwing up 500/ 503 errors and I can't work out why. I've tried a lot of things.

Going through the error logs, I've had found a couple of fatal php errors, see below. BUT these are very occasional (once a day) and the slow loading/errors are constant.

PHP Fatal error: Uncaught exception 'ADODB_Exception' with message 'mysqlt error: [2006: MySQL server has gone away] in EXECUTE("SELECT TABLE_NAME, CASE WHEN TABLE_TYPE = 'VIEW' THEN 'V' ELSE 'T' END FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA=SCHEMA()")
' in []/concrete/libraries/3rdparty/adodb/
Stack trace:
#0 []/concrete/libraries/3rdparty/adodb/ adodb_throw('mysqlt', 'EXECUTE', 2006, 'MySQL server ha...', 'SELECT TABLE_NA...', false, Object(ADODB_mysqlt))
#1 []/concrete/libraries/3rdparty/adodb/ ADOConnection->_Execute('SELECT TABLE_NA...', false)
#2 []/concrete/libraries/3rdparty/adodb/ ADOConnection->Execute('SELECT TABLE_NA...')
#3 []/concrete/libraries/3rdparty/adodb/drivers/adodb-mysql in /home/wgsp26q0z9kb/public_html/ on line 78

A second error in the logs is:
PHP Fatal error: Cannot unset $this in []/concrete/libraries/3rdparty/adodb/ on line 178

There are a couple of exceptions thrown up as well, but again, a total of 2 since the problem started.

A few minor changes had been made to the site around the time of the problem starting: minor text edits to the homepage and 2 problog posts (changes made by client) and I published a new xml sitemap, and ran a couple of other jobs through the dashboard. (including page index all, which did run forever,<a href=""> until I tweaked the database to stop it</a>)

Looking at the server usage in cpanel: processes and physical memory use have gone up, while cpu use has gone down. (all well below hosting company limits)

I've been going through the forums, following various bits of advice: clearing caches, truncating the pagestatistics table, (Db now around 17mb). Nothing has worked so far.

I'm baffled and would be so grateful for any help or advice.

View Replies:
ConcreteOwl replied on at Permalink Reply
JohntheFish replied on at Permalink Reply
Is this something that has suddenly started happening, or something that has been slowly creeping up on you?

If sudden, does it correlate with any of:
- c5 core update
- package update
- host PHP version update
- host MySQL update
talia7 replied on at Permalink Reply
It was very sudden, and as far as I can tell the timing didn't coincide with any of the things on your list.

The problem seems to have resolved itself for the moment, but I've thought this before and it's resumed the following morning. I'm keeping a watch on it for the moment, but it would be great to have some ideas about where to look for causes if it happens again.

I'm wondering if the cause is something to do with the page search indexing job...? It's a largish site (500+ pages) and took some time for the index to update.