1) Welcome to the Party!
2) As you are obviously new here, I have some suggestions to help us all help you:
When you post, please post your entire configuration including (but not limited to) your installation method and vicidial version with build.
This IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "from scratch" you must post your operating system and should also post the .iso version from which you installed your original operating system. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) If you have 150 users and have not taken the Vicidial course in Clearwater, you should seriously consider it. There are several enhancements and fixes that may be required to get your system stable, but knowing only how many servers you have is not likely to get us there. LOL
4) run "htop" on each machine during an occurrence and look for overload.
5) Look also for slow queries in the mysql server. Look also at any customizations you may have that could be requesting mysql information and locking tables. Also consider some of the recent posts regarding mysql overload from too many connections, there has been some progress on that front for heavily used servers with some slight adjustments (if mysql turns out to be an issue).
6) Happy Hunting!