Ping-pong extension

Hello all.

Decided to rebuild my TB 2.6.X.X as it was acting a little weird and now something even weirder is occurring with a remote extension (ATA). Here's the error I get:

[2011-12-14 22:05:03] VERBOSE[2146] logger.c: -- Registered SIP '900' at 74.58.XXX.XXX port 61272 expires 3600
[2011-12-14 22:05:03] NOTICE[2146] chan_sip.c: Peer '900' is now Reachable. (83ms / 2000ms)
[2011-12-14 22:06:07] NOTICE[2146] chan_sip.c: Peer '900' is now UNREACHABLE! Last qualify: 83

The 2000ms would explain the de-reg. The extension registers and one minute later drops, then 10 minutes later it registers again, one minute later it drops, 10 minutes later it registers again, etc... the whole time I am getting an average 70ms ping from my server to the remote extension's IP.

Any ideas?

Thank you