If your Variance is doing a saw-tooth pattern, then it is most likely spyware/malware programs. Anti-virus software does not catch those programs.
Typically the sawtooth is caused by an application running, taking away from the game cycles.
In your "Internet Options" (Control Panel), check your "Security->Internet", if you have "ActiveX" enabled, then it is probably spyware as that is the primary delivery mechanism for spyware programs.