Dropping calls after exactly 4 minutes

Hi all.

I’m having some weird problems with my FreePBX setup…
All calls (but not between extensions) are being dropped without any reasons after exactly 4 minutes.

After a week of trying everything, I can’t really find out, why are calls dropped, because there isn’t anything written to debug log, after call drops…
Just “Locally bridging SIP/54-00000018 and SIP/Amis-00000019” and then “-- Executing [s@macro-hangupcall:3] Hangup(“SIP/54-00000018”, “”) in new stack” after call drops (user hangs up few secons after that happens).
I noticed, that phones don’t detect, that call was dropped - call duration timer still counts.

I’m, using Intel Atom mobo, with integrated gigabit Intel ethernet, and 4-port DLink(sundance) PCI NIC.
Intergated NIC is connected to LAN switch, and then to 6 Cisco SPA303 phones, and one Cisco PAP2T adapter. No NAT or anything. Just a small office network…
One port from 4-port card is connected directly to SIP provider (DSL modem).
Second port directly to another PAP2T adapter.

Using latest and fully updated FreePBX.

Any ideas/hints welcome!

Some atom boards with linux have a faulty NIC driver. Can you ping the server when the calls drop? type sip show peers, what are the qualify times?