Polycom sets go lagged for no apparent reason

I’m monitoring all phones with Nagios. The check script verifies each phone is registered and responsive via “sip show peer”. If I don’t get an “OK (XXms)” response in each check, Nagios raises a red flag. I check each phone for a heartbeat once per minute. The site has a combination of 430s and 501s, though the problem is not model-specific. All are connected via a 100MBit lan. There is also a Snom 360 connected in the phone pool aside two SPA-2002s, no lag problems there.

I know the network isn’t the issue as there is no ping latency to speak of (always <10ms response time). However, even when a ping has just returned as minimal for a certain phone, sip qualify time can be as much as 4 seconds for the very same unit. Therefore, I’ve had to set qualify=5000 to keep the phones from being tagged as LAGGED and forgotten momentarily by Asterisk should a call come through. In a moment or two, always in less than a minute, the phone is again flagged REACHABLE.

The phones are performing well otherwise.

Polycom firmware: v2.0.2
Asterisk: v1.2.13

Is this happening to you? Any config changes you’d recommend?