100% CPU when control clicked in mix window

[kade]

Chipset: nVidia nForce 410/430
Processor: AMD Athlon 64 3500+ 2.2GHz
RAM: 2GB
OS: XP SP2
Video: nVidia GeForce 7300GS
PT: 7.1 + latest patch

I have a 6 audio track session. If I start playing, open the mix window, and then click a slider to adjust a level the mix window freezes along with the edit window. Audio continues to playback normally. Sometimes the slider will move freely without freezing or will become 'jittery' moving in steps (as do the meters, with each click). All this time the audio continues to play normally.

If I run Windows Task Manager I see that before/after I open the mix window the CPU is at around 5%. When I simply click once on a slider it jumps to 100% and stays there until I close the mix window, at which point it returns to the lower level.

What is odd is that at other times the mix window appears to behave normally. I've followed the setup recomendations and tried adjusting the Playback Engine settings but nothing appears to change.

Has anyone any suggestions??