About a month and a half ago, one of my guys worked with Microsoft to diagnose a problem within Windows 2003 SP1 which prevented timer events from being signaled under severe loads. Check out the knowledge base article here for an explanation: http://support.microsoft.com/?kbid=900822. I just ran into the same situation while deploying at a customer site so thought I'd post this to keep you from the pain of diagnosing this very tricky problem. I strongly encourage everyone that plans on using iMotion or other applications that use System.Threading.Timer on a Windows 2003 Server SP1 machine to contact Microsoft to obtain the hot fix before deployment.
Note: If you're running iMotion on a Windows 2003 SP1 machine without the hot fix applied and are experiencing what appears to be dropped tags then you'll likely benefit from the hot fix so obtain it immediately.