Successfully connected clients disconnetcs after seconds

Question

Hello,
I'm having troubles with AP541N in clustered mode.
The problem is that clients connects successfully but get disconnected  after seconds without any visible reason.
The AP541N is configured to authenticate the clients against a Symantec Network Access Control (SNAC) and a radius server.
eventlog from radius server:
"User xxxxxx was granted access."
SNAC log:
Client[0000000c] xxxxxxxxxxxxxxxx, Status Recevied(HI:PASSED, EAP:PASSED, PRO:PASSED), UID is CORRECT, Enforcer matches(HI:PASSED, EAP:PASSED, PRO:ANY), switch VLAN XXXX on switch nnn.nnn.nnn.nnn.
client settings (Windows 7 32-bit):
WPA2-Enterprise/AES
EAP-Typ: secured EAP (PEAP)
This problems happens not to all clients (all clients are Dell Latitude laptops with Intel Wifi, but different models).
When I use eg. a D-Link USB Wifi adapter in a laptop causing troubles, the connection will work fine, no disconnects.
I have tested with Intel driver version 13.4.0.139
When I connect a laptop causing troubles to another access point (eg. D-Link) it will work fine.
Any ideas to solve the problem are welcome.
AP541N Device Information 
Product Identifier: AP541N-E-K9 
Hardware Version:   V01 
Software Version:   AP541N-K9-1.9(2) 
Serial Number:      DNI1422A06V 
Device Name:        AP541N-E-K9 
Device Description: 802.11n Dual Band Access Point - Single Radio 
Wolfgang

Answer 1

Dear Wolfgang,
i have the same problems with old dlink ap-s.
After change to new hardware - no more, works fine.
mfg
Best regards.