Cisco Wireless :: WAP4410N Not Transferring Anything

Clents:

1) HP nw8440 with Intel PRO/wireless 3945ABG, running Vista Business, all latest driver & Win updates (ok, missing the last few that should arrive soon);

2) Dell m4400 of some sort  (supports 802.11n) running Windows 7 beta. This user says wifi works flawlessly in his home environment.
 
Transfer type: not even transferring anything - just sit there watching it... pings to otherwise speedy servers up and down like a yoyo followed by drop of ping transmission. At one stage I even saw the unit not responding to pings or admin website directly off LAN port once the problem had occurred - all leds looking normal, but no ICMP response. power cycled the unit, and it all started working again.
 
Only AP in our network - others in neighbouring buildings, but not under our control.
 
Can't say I've verified no overlapping IP's. will check tomorrow.
 
Re logging, I have syslog going off to neaby Linux server... despite having all the log types enabled, the only types of messages I see being logged are those about clients trying to connect, and options being changed. No sign of any error messages.
 
Ours is also a straight forward network - no Vlans, just one SSID, WPA-Personal/TKIP. Thought it might be to do with MAC filter, so disabled that, but no improvement. Tried channels 6 & 9 so far. I know in years gone by there has been plenty of talk re interference from microwave ovens etc - any chance of this being the culprit? Haven't toyed with any of the options on the Advanced tab under Wireless.
 
Tried switching back to BG instead of BGN (the Dell has N support) but that didn't work either.
 
Is there any chance this has anything to do with Vista/Win7 compared to XP? I've not tested with an XP client OS, but the somewhat vague report from a more distant XP user on the network was that he wasn't having problems at around the time we were, though I'd need to probe him more to be sure.
 
The AP was purchased around 1 week ago, and came with 1.0.12 (latest) installed. I also did a factory reset upon first use, before configuring. WPA key has 13 charcters, being a mix of numeric, alpha (both cases) and special (!@#$ etc...) characters.
 
This is decidedly embarassing for us since we were finally able to ditch the old Dlink AP we hated so much - but so far this Linxsys/Cisco ain't faring much better. Bring on 1.0.13 I say... :)
 
Edit: the unit is in regular/default AP mode, with own IP being static (SBS DHCP server has that MAC hard coded to that IP also). I presume teh AP's MAC is the same whether on WiFi or LAN port (??).