Poor network file transfer speeds *overall*

Ok got a major networking file transfer issue when transfering ~2+gb files to a windows server machine, to start - here are the specs:

Server:
Intel Celeron 420 (1.6ghz)
2gb DDR2
WD 160gb (OS HDD)
3 x 1tb WD GreenPower SATA running off a HARDWARE RAID5 CARD (Shared Folders)
Intel Pro/1000 PCIe 1x GBe network card
Windows Storage Server 2003 R2

Client Machine:
Q6600 @ 3.5
8gb ram
2x250 RAID0
1x640gb
Realtek GBe (onboard)
Vista Ultimate SP1 (with all updates)


Now when i send files that are larger then 2gb the transfer will start at 90+mbps but after about ~1.5gb it will suddenly slow down to ~5 mbps at best, as if the network transfer from my Vista machine gets buffered to my servers ram and floods that till it runs out of ram and just ends up thrashing the HDD's (even when the transfer is finnished the hdd led's still flicker away). See image below of network transfer graph.



Things iv tried with no change what so ever:
Disabled volume shadow
Disabled write cache to all hdd's
Enabled write cache to all hdd's
All sorts of registry tweaks (paging etc)
Change system priority to foreground and background tasks
Enlarged the page file (not even used - hdd lead for OS doesnt even flicker etc)
Disabled the page file
Ethernet - different drivers (client and server), force modes like 1000TX etc

I need some new ideas on how to solve this issue! its just write issues - driving me nuts! :S

and the RAID5 card is an adaptec 2610SA (same as the more common 2410SA) - im starting to hear bad things about them :S

Yup OS drive is fine, so yeah you might be right there - raid card perhaps, but this is the thing - got 2 servers that do it! My other server - E5200, and a RocketRaid2300 (not totally hardware) - exactly the same issue.

Its pissing me off big time

The arrays are totally build+verified, reads fine but wont write well at all.


Iv had a freenas server read/write at 50mb/s without hassle in *software* RAID5 before i went to Windows Server and a hardware raid card (and the other, the rocketraid 2300)