Why does my audio engine sometimes run slow ?

Hi,
A lot of times when I fire up Sonar (Platinum) and I have a track running at, let's say 130 bpm
it goes slow perhaps about 90 bpm, I then have to switch off my PC, and reboot it, but it
still sometimes runs slow, then it will go back to normal, any ideas ?
 
Thanks
John 

PS: Windows sounds or another program maybe taking over the audio device and changing your samplerates which would explain the sporadic/erratic behavior.