1-2 second audio dropout

I have been experiencing a significant problem with some of the Asterisk Installations (on more than one) where during a conversation the audio will ‘dropout’ for 1-2 seconds, but only one side. This can be a call either using an IAX trunk (to another PABX) or to a PSTN (Zap) channel. It is possible

The audio drop out only occurs on one side, and occurs with all versions of Asterisk that are being used. Currently 1.2.17 and 1.4.2. It also occurs with earlier versions.

I’d appreciate any feedback on this issue.