Hi,
Do you see any errors in the server access logs when the monitors fail? Do you have connection limits set for the pool members or node addresses? Which LTM version are you running?
I added some monitor troubleshooting tips to this page:
http://devcentral.f5.com/wiki/default.aspx/AdvDesignConfig/TroubleshootingLtmMonitors.html
If you see anything that could be added to the page, either add it or reply here with comments.
If the issue only occurs during a (light) load test, it will be fun to pick through the logs and/or tcpdumps to find the problem, but it should still be possible to troubleshoot this.
Thanks,
Aaron