cluster ip address not responding

i have exchange2010 mail server hosted in tow physical machines

in the first machine there is mail box server and CAS/HUB1 visual machine (hyper-v)

in the second machine there is EDGE SERVER and CAS/HUB2 isual machine (hyper-v)

mail box server ip 192.168.1.14

CAS/HUB1 192.168.1.23

CAS/HUB2 192.168.1.24

there in NLB network load balance for CAS/HUB1 and CAS/HUB2 and the cluster ip is 192.168.1.3

my problem is the cluster ip some times the ip stop responding for long time a