Weird Network Slowdown

I have two identical computers, one with Vista64 and the other with Win7-64. 4GB of ram, i7-920s blah blah blah.

Both have gig Intel integrated NICs with full IPv4/6 TX/RX TCP/UDP offloading. Both are connected to a Netgear WNDR3700 4port gig router. I have 15' CAT5E cables connected, NICs are both showing 1gbps and router/switch is lighting up as gig.

Now on to the weirdness. A few days back I decided to copy over some files to my wife's comp. They only copied over ~15MB/sec. I wanted to find out where the bottleneck was, so I downloaded Iperf.

Iperf said I was getting 941mbits/sec and if I ran two at the same time, one in both directions, it would list one as 460mbits and the other at 940mbits. So, I was getting near 1.5gbit/sec at full duplex. I researched a bit and a few sites said to disable Differential compression. I disabled and and suddenly I was getting 110MB/sec transferring files. This was good.

Now, I don't shut down my computers. I got on my computer the next day and went to copy some other files over and I was getting ~1MB/sec. I was like WTF?! So I did an Iperf test. 9mbits......

I've tried restarting both computers, installing the newest NIC drivers from Intel, replugging in the cables, double checking all of the status lights, disabled firewalls, no anti-virus/anti-spyware/etc running. No difference at all. I can go download files off the internet at 30mbit/sec on both computers, but I can't communicate between the computers faster than 10mbit.

I tried using my private IPv6 for Iperf and I get the exact same results

edit: Same results with UDP and Iperf.

Iperf says 0.000ms avg lag and 0.000ms-0.013ms of jitter, 0 packetloss

edit2: The Intel NICs on both computers have a diagnostic check for both the NIC hardware and the cable. Both turn up fine. The NIC is able to communicate with itself at full 1gbps.

I tried a different network cable and connected both computers directly and I got the same Iperf results. This rules out cable/NICs/Switch.

My guess is Windows is f'n something up good. Since I get the same issue with UDP, that rules out any TCP or filesharing protocols.

/sigh.. got it

Full Duplex
[192] 0.0-10.0 sec 913 MBytes 764 Mbits/sec
[168] 0.0-10.0 sec 916 MBytes 768 Mbits/sec
total 1,532mbit/sec

One way (half-duplex)
[152] 0.0-10.0 sec 1.10 GBytes 944 Mbits/sec


stupid command

netsh interface tcp set global autotuninglevel=disabled

I didn't think of it before because my UDP was also affected and this command SPECIFICALLY says for TCP and all of the documentation says it's for TCP...... I guess it somehow affects UDP also.

oh-well, it only "helps" high bandwidth high latency connections anyway.