asterisk users - Dec 2006 - calls interrupted by music on hold

Hi,
I have Asterisk 1.2.9.1 on a Debian box with a beronet BRI card 
(install-misdn-mqueue driver). Sometimes, calls are interrupted by music 
on hold without any reason: the caller and the callee are put on hold 
for few seconds (they both listen to moh) and then the call is 
established again....and it happens more frequently during long calls. 
Asterisk logs report nothing.
Anybody has any ideas about this problem?

TIA

Giorgio Incantalupo