Really slow file move/copy operations on WS2008, with lock-ups

I've got a Windows Server 2008 box that has serious issues when copying or moving anything larger than 400MB or so.

The server is an Opteron 1210 running Hyper-V with the file operation on the root partition (so the HDD isn't virtualised). The single virtualised server instance does have HDD activity, but it's about a megabyte every minute or so.

The HDD hardware is two identical 750GB SATA HDDs in RAID1 configuration, controlled by an Adaptec HostRAID controller built-in to the motherboard (an ASUS M2N-LR/SATA).

The HDD has all the performance checkboxes checked off as well.

Small, short bursts of copying go by without issue at your regular pace of around 50MB/sec or so. But if you try to do a continuous write operation bigger than 400MB or so (I don't have an exact figure) the transfer speed slows down to around 8MB/sec and then the system becomes totally unresponsive (including the virtualised instance) whilst the HDD activity light is constantly on. If you click on the Cancel Copy button and leave it alone for a few minutes it'll unfreeze and everything's back to normal.

I can't tell if the fault is with Hyper-V or my HostRAID configuration. I'm tempted to lean on the HostRAID since it isn't a true $800 RAID controller (the motherboard itself was £200), but would a "Fake RAID" like this cause the problems as described?

My motherboard has a Zero Channel PCI-X slot on it; if I got a compatible card would that alleviate things... and I'd need to rebuild the RAID array, wouldn't I?

Oh, and the RAID1 array holds everything, The slowness also appears when copying from one drive to another (I just added in a 250GB containing ripped TV shows and copying them onto the RAID1 takes forever due to the symptoms described)