Audio clips some of the time

Just like the title says. I am using only instrument tracks, and midi track for Battery. The tracks use stuff from komplete 6: kontakt, battery, guitar rig. Some other misc plugins: voxengo stereotouch, vintage compressor (steinberg), and studio eq.

The problem is that sometimes the track will clip, and other times it will not. If I just loop the thing, it seems that it clips maybe 30% of the time.

I have no idea what would cause this. Should it not clip always or never? I am not changing anything at all. I would think that if it clipped, it would do so 100% of the time. Also when it clips, it must not be by much because I don’t hear that nice pop or distortion that happens with clipping. Shouldn’t the track play back exactly the same every time?

I searched the forums, but I am not even sure what to look for. Any help or ideas would be greatly appreciated.

cubase 5.5.3
i7 930
m-audio firewire 410 (I know, believe me I am saving up for better)
win 7 64
6 gb ram
novation remote 37 sl (plugs not wrapped, since this crashes cubase, just used as a standard midi controller)