Try to give some more information;)
Server OS, Wowza Version, Java Version, Server setup.
Looked in the logs? which logs (wowza logs, serverlogs)?
i think the first thing you should at least do is monitor the server itself. cpu load, memory usage, network load and traffic. So from there when you have date/time of the issue you can check if you have a high load or something else.
Some tools for server monitoring: nagios, zabbix. (zabbix has an easy setup and good graphical interface)
Other thing what ive seen before with buffering issues.
The upload of the stream is too high compared to the internet speed of the customer. It will max-out the upload, which will cause buffering issues on the client side.
Even your disks can be a problem. If they have cluster errors, or other perfermance errors with reading/writing it can also cause issues (i dont know how you’ve setup your colo server).
You have some work to do ;).
Does it happen on recurring moments?
Does it bufffer also with VOD or only live streams?
Does it happen with multiple streams or only with one stream?
Are your disks in good state?
And live monitor your server (cpu, memory, disk, network)