Please remember to always list the FULL vicidial version With Build. Makes a difference for everyone (not just you) to link those with similar problems using the same build ...
Onward!
When your system overloads, it can cause mysql to overload. Mysql may or may not be the actual cause of the overload.
If your system has been in use for a long time, you should be archiving (and perhaps pruning) your log tables. There are processes in the /usr/share/astguiclient perl scripts folder for this purpose. They are also usually included in crontab -e but commented out for ease of activation. Read the comments in the scripts for archiving to see what they do. Some of the log tables have no value if you are not troubleshooting and others are actually used in Reports, so don't just go nuts and start deleting entries.
If any of your database tables have millions of records, it's entirely possible mysql is beginning to have troubles with containment. If any have Tens of Millions ... even more so. Look at how to reduce those table sizes.
Next up: there's a mysql configuration file aptly named my-big-vici.cnf (or similar). It has settings for high-volume systems which may come in handy.
You can check active queries during overload times to see if there are specific queries causing issues (such as time zone lookups or filters). "show processlist" is a very handy mysql command. You can also use phpMyAdmin or mytop or mtop (or workbench for mysql).
All sorts of things can overload Vicidial. But one of the biggies: 40 live agents autodialing on a single server. You may need to Cluster your system. Which makes it even more likely you'll want to use Vicibox for your next install! It clusters automatically.
(Thanks Kumba!)