I don't have a cheat sheet of what 5.1.x.x version is 5.1 SPx, but I do know that 5.1, 5.1SP1, and 5.1SP2 had some issues with memory leaks and high CPU usage, I had to upgrade about a dozen 5.1SP2 systems to SP4PR5 a while back due to similar issues and VM lockups. I would recommend upgrading to latest 5.1 release on the old Inter-Tel FTP site, if you don't know to access it, give tech support a call. I believe 5.1 SP4 PR5 was the most current last time I looked, something like 5.1.0.52