Problem with Pro Tools audio clock

[alacart5]

hey guys, I have this problem. Not always, and not never, when I start up or open a session and press play, PT will deliver this error message:

"Problem detected with audio clock. Check that your clock source and sample rate are correct." and not play.

I then need to restart pro tools over and over until magically for some reason it stops doing that. Sometimes it does this and sometimes it doesn't, which is the biggest thing i don't understand.

I am running PT LE 7.1, with the cse update, using an mbox2 on windows XP. All my sessions, when working, are at 44.1 kHz, 24 bit, using S/PDIF clock source. Also I am using it on a computer that I built myself with a Dual Core Athlon Processor.

Let me know what ya think.

[Foreverain4]

do you have anything connected to the spdif input? if not, change your clock to internal and leave it there.

[pharealprince]

my clock source says see control panel and this might seem stupid but which control panel