Problems during some calls

Dear all,
I have installed Trixbox v.1.2.3 and I use one TDM400P with 3 FXO cards to connect to PSTN lines. I have a problem (randomly) when my call continue for several minutes because I have some voice holes (2-3 seconds) during the call.
I use three types of hard phone: Grandstream 2000, Linksys 941 and Cordless Siemens C450IP.
Any ideas? Where should I start looking?
Thanks in advance.