VM's are not pinging until it is migrated to another ESX host

Hi

I have some virtual machines in which suddenly are not pinging, but if you go to the console the OS is up ( but even inside the VM with a local account , if I try to ping to another server in the same ESXi is not pinging) , and the only way to start it again to ping is migrating it to another ESXi Host or even shutdown the VM,  anyone know why this is happening?

ESXi v 4.0

OS = 2003, 208 R2 , mostly

It can be with any of the network cards:  E1000 or VMINC3

Thank you!!!!