Problem with audio clock

[El Rico]

Greetings. I'm running Mac OS X 10.5.4 + Pro Tools LE 7.4.2 + MBox 2 Pro. The problem is I cant' record a damn note ! "Problem detected with audio clock. Check that your clock source and sample rate are correct", but they are ! For example, I plug my bass directly in Input 1, create a mono audio track then press record once: I hear my bass through my monitors but there's no signal in the track! A friend of mine gave me a "tip": he told me to change the clock source from Internal to S/PDIF. Then return to Internal after reading the error message. Now it's working and I'm finally able to record. But sometimes this advice is useless and I have to restart Pro Tool again and again and again until it works. Maybe I should send back my MBox for check-out and repair...Please help me, I'm desperate !