Hosts that won't stay connected to vCenter

ESXi 4.1.0 and vCenter 4.1.0

We have four hosts in a cluster, two have no issues while the other two will not stay connected to vCenter.  I have checked everything in KB article 1003409 (http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1003409) and found no issues.

All four servers are IBM x3690 X5 and all are connected to the same four SAN LUNs, the only difference is that the management interfaces for the problem servers are on a separate subnet, but port 902 UDP is opened between them and the vCenter server.  I can manually reconnect them but they will disconnect again in a minute or so.

Does anybody have any ideas beyond what is in article 1003409 that I can check?

Thanks,

Darryl