VMware ESXi 5.0  constantly resetting vmnic

I can't stay connected to my ESXi 5.0 host or its VM's, via either vSphere client or Windows RDP. The VM Kernel Log reports errors like:

1836 Netsched ... packets seem stuck... issuing reset of vmnic2
1817 ... scheduler [<some number>] lockup [stopped=0]...
1827 ... detected at 578999 while last xmit at 573549 and 27/20375

I did try to optimize the TCP stack (using TCP Optimizer 3 from http://www.speedguide.net/downloads.php) in one of my Windows VM's then did a host reboot (for other reasons) just before this problem appears. Could that have corrupted something or is it just a coincidence?

Tried removing all other devices from my router except ESXi host and a laptop to run vSphere client and RDP. Was able to shut down all VM's to keep them from causing or contributing to the problem. Tried adjusting router MTU (between 1492 and 1500) guessing that is was a packet fragmentation issue (although the router should be able to handle that I thought). Tried restarting all devices (host, router, laptop). Only thing I haven't done yet is re-installing ESXi OS on the USB thumb drive.