New to Freenas - weird perfomance issue: wirelss faster than wired?

[goerz]

Hi, I'm new to Freenas and to this forum and I have a strange network performance issue. I've just built myself a Freenas 8.02 system with an Atom D525 mainboard (Asus AT5NM10T-I) with 4Gb of ram. I know it's not a powerful system for zfs use, but noise is an issue for me and I needed a fanless mainboard. I installed 4x2Tb Seagate "green" drives configured in raidz-1. Everythings works well, but I have performance issues over my network. I have a desktop pc connected to the nas through a Gigabit switch, and I also have a wireless G network. The desktop pc is a liquid cooled Win7 64bit machine, with an i7 2600k Intel processor, an Asus P8P67 mainboard and 8Gb of Ram, recently built, that has never given me any perfomance issue before (I use it mainly for gaming and photo and video editing) . The problem is that when I try to listen to music or watch a video stored on my freenas system from my desktop I get numerous dropouts and stuttering performance. Reading/writing speeds from the desktop via the cifs shares are variable, from 80Mb/s to 10Mb/s, depending on whether the freenas system is performing other read/write tasks at the same moment or not. However, when I stream wirelessly the very same audio an video files to a Win7 64bit laptop (cifs share) or to a Linux Voyage box (nfs share) I use as a music server, I don't get any performance issues: playback of high resolution audio and video files is flawless, even when streaming different files to the two clients at the same time.
Initially I thought I had some issues with the Realtek 8111E ethernet interface of my freenas system, but I tested the connection between the desktop and the nas with iperf and I got a 847Mb/speed, so I the bottleneck must be somewhere else, probably in my desktop system.
Any hints on how to troubleshoot this problem are welcome.
Thanks,
Goerz