que? what's happening in audacity

Hi

I have made a drumtrack in hammerhead and streamed it to the disk.

I imported it in Audacity and recorded a lot of stuff over it, before I trimmed it here and there. Everything worked fine. I exported the file as mp3 and everything fine still.

But then I recorded something more and suddenly parts of the drumtrack didn't ...ehr... make any sound. I can see from the frequencies - it is still there - but the sound clips of in the middle and comes back in the end...

I imported it again - trimmed off the parts I don't need - and after some recording parts of the drums disapeared again. The same thing happend for the third time right now

Did I make myself clear?
e.g. here:

When I play only the track shown - the first two beats are fine, but then it stops even though it is there - see?

any ideas?