Intermittent phone quality issues

Hello, I was just wondering if someone could help me troubleshoot an issue. I am currently running elastix 1.6 with FreePBX cor The problem we are having is that phone calls are flaking one minute it is working fine the next it is cutting out. We are using a lenovo pentium4 machine with 2Gb of RAM with a Digium FXO card with 2 vonage lines and also are using a sip trunk from broadvoice it does not appear to be a service issue because the problem is on all line no matter which one we use. We have been using this system for about 8 months now and have just started having more issues within the past month. If someone could please let me know what info I need to get and how to get it to allow troubleshooting. I am not a phone system guru and have not had to troubleshoot issues like this before any help would be appreciated thanks.

sounds more like a network issue. are your extensions sharing the LAN with heavy data usage [e.g. video streams, high use database servers] etc. I have seen this kind of problem when the phones were on the same LAN segment as a group of graphics designers who were storing 10GB files on a NAS server.

have you implement QoS/ToS to get your voice payloads higher priority through the network infrastructure?