Vista - Need help; gigabit network transfer speeds

Hey..new to the forum and short on ideas with a strange issue.

First, the setup and application...I frequently transfer quite large files (20-40GB) from my primary system (Vista Ultimate 64bit, evga 680i mobo w/onboard gigabit NICs) to a PC used as a home theater PC (Vista Home Premium 32bit, asus P5E VM-HDMI, onboard NIC). I recently wired my home with cat6 and installed all gigabit hardware for distribution (Linksys WRT610 router; D-Link DGS-2208 gigabit switch) to improve network transfer speeds. All HDDs are 7200RPM SATAs

My issue is this...network transfer rates average only 15MB/s from the primary to the HTPC--not even close to what I expected. I've searched the net for suggestions, but several fix attempts yielded no improvement (diable IPv6; enable/disable onboard NIC; replace NIC).

I thought I had solved the issue this weekend when I found a suggestion to disable Vista autotuning level using the following in an elevated prompt on my primary system:

netsh interface tcp set global autotuning=disabled

After a reboot, I attempted a file transfer and BOOM--75MB/s! I fixed it, right? Not exactly...somehow after a few hours--with no other system or network changes--the transfer rates again dropped back to ~15MB/s.

I'm baffled. So I try something that probably makes little sense (I'm not very adept at network troubleshooting) and set my MTU on the Linksys router to 'Fixed' at 1464 instead of 'auto' (I have ATT DSL, and had tested my connection previously with 1464 being the optimal MTU). This I thought maybe did the trick on Sunday, but again this morning my transfer rate is back down to 15MB/s.

I'm not optimistic that theres an easy path toward troubleshooting and fixing this, but I'm hoping for suggestions on where to go from here.

Can anyone who finished reading this chime-in with some ideas? I'm fresh out

Do you have Vista SP1 installed? Vista without SP1 is known to have file transfer issues.

Also, do you have other machines you can move data between? If so, does it occur on them as well?