esxi5 hosts are being randomly "disconnected"

Hello,
We started to face strange issue with our setup. We have 5 esxi5       servers with the following config:

vm1 - has 4 network cards:
1 - regular LAN switch, management.
2 - regular switch, ISCSI
3 - dvSwitch - LAN, ISCSI, FaultTolerance
4 - dvSwitch - WAN

vm2, vm3, vm4 - connected into cluster for vCloud. all 3 servers       have the same network configration, as vm1, except that they have       vShield enabled, while vm1 - does not.

vm5 - is not part of cluster or any distributed switch, server has       just 2 network cards.
1 - LAN, management, ISCSI
2 - WAN.


For last week or two servers randomly become "disconnected" in       vCenter, although physical servers are still up and running, none       of running VPSes are affected there - so basically it is just       vCenter that "looses" servers. I am able to connect to each server       separately with vSphere client, also able to ping them. After some       time (10-30 minutes) I am able to connect server to vCenter.

One more note - physically we have 2 separated networks (WAN and       LAN). so basically 3 cards on first 4 servers are connected to the       same physical network (might be connected to different physical       servers).

Can anyone please give me a good advice - what could be wrong and       what would be better way for setup and redundancy?

Regards,
Anton.