Some background task is monopolizing your system resources. I doubt it's the synth, or any other thread within the DAW. That it goes away while running Task Manager is troubling, as that is behavior a clever virus might exhibit.
Download a copy of Process Explorer from Sysinternals if you don't already have it. It will show far more information than Task Manager. While you're there, also grab a copy of Autoruns to see what's automatically starting up when you boot. Another good sleuthing tool is CurrPorts from
Nirsoft, which reveals all your network connections and what processes are using them.