signal autometic disconnecting after 15 to 20 min, setup " ZD1200, r720 with 802.3at power"

[harekrishna_mon]

while using r720 and ZD1200(ver- 10.1.0.1553) as controller and powering AP's with 802.3at.
the ap's are placed in instute where at a time around 60 students connect to the access point 
But 15- 20 min of using Wi-Fi singnal went after a few seconds it's comming up,
what could be the reason behaind this signal flactuation?,

some info:
we are receiving 18 dBm TX power for the r720  
DHCP server we are using is SOPHOS XG430 ( Checked with diffrent DHCP server as well)

Please help with the same

Thanks,

[harekrishna_mon]

We selected all, channels for 2.4 GHZ