I have checked plantronics documentatoin for DECT. Apparently 1.90-1.92 GHz. Also I have some kind of frequency hopping across 2.4GHz, and no real idea of what that could be except my spectrum analyser is nearby some bluetooth devices - I guess I was after a second opinion. I get sudden drops in signal quality on 2.4GHz, and was wondering if anyone else had similar experiences.
I have seen this problem. Firstly, the spectrum analyser you used; was it one that was built into the AP or a stand-alone unit?
The DECT interference on an AP can be very tricky to find. This is because, as you said, it's not in the 2.4Ghz spectrum. The problem with this, and other out-of-band interference, is that if it goes into the receive chain of an AP that doesn't have any filtering for its band, then that signal bounces around the RF circuitry and then its harmonics become evident across the 2.4Ghz band. You would probably see this on an internal AP spectrum analyser (same receive path as operational AP), whereas an external one like Airmagnet or WiSpy wouldn't show it because they are filtered for their band.
Filters should be part of any receive system. Here is a little video I made with a guy from ACMA as a demonstration. For all the Americans on here (I guess most of you) ACMA is Australia's version of your FCC. He has an external adjustable filter which shows the difference between having one and not having one. The spectrum analyser is showing the 2.4Ghz band.This was taken in the public outdoor area of the Australian Open tennis tournament, where there was a GSM town about 200m away. Add the high density nature of unknown public devices and you've got fun all around :)
In addition to my last post, to verify if it's the problem, simply see if you can turn the DECT system off and then see if the problem goes away.
I also know that's easier said than done in a lot of places where they won't want to do that. Good luck.