Windows 7 - issues with media files on network drive

I'm having some odd issues opening media files (large MKV's) over mapped network drives. Regardless of what media player I use (XBMC, VLC, MPC-HC) the playback pauses every few seconds, apparently to re-fill the buffers. Odd things is both computers are very capable (intel I7/I3 with fast HD's) not busy doing anything else, and they're connected by gigabit ethernet.

Doing a file copy gives me full speed - no problem. Copying the file and playing it locally - no problem.

What puzzles me further is that the HDD led on the computer with the file, isn't even flashing half the time - not even when the remote "client" is experiencing the buffer underrun. It's seems it starts reading the file when I open it remotely, but then stops after a few seconds.

What. the. hell. ?

This is regardless of whether the file resides on an internal SATA HDD, or an external USB drive.

I've googled and examined my settings for transparent offline files settings etc, but I haven't been able to find any problems - but maybe I'm looking at the wrong things.

Help?!

Thanks for the suggestions. All A/V is turned off or uninstalled - same with firewalls. Changed network security from 128bit to 40/56. No difference.

I have to assume that it's somehow related to some caching mechanism on the client computer (the one trying to open a remote file) - as even after a reboot, it will very prompty start playing the video file, without any HDD activity on the computer which hosts the file. And this is regardless of player software - they'll all start playing the first few seconds flawlessly then pause playback (some with artifacts) and then VERY slowly rebuild a buffer (still seemingly without HDD activity on the host computer), only to play a few more seconds and then pause again. Eventually the host WILL display HDD activity, but the pattern remains - a few seconds of replay, then a large pause.

More ideas?

The networking equipment is a ASUS RT-N66U router, and I open the files in VLC/MPC-HC (via the mapped network drive). And in XMBC it's via SMB.

BUT - I might have solved the root of the problem without noticing which "tweak" solved it. I simply kept focusing on that one file because it had the highest bitrate, but it SEEMS as if somehow at some point a broken cached version of this file got created, and I'm STILL getting that version. I've now tried a few other high bitrate files and they now appear to stream fine (for several minutes - and they didn't when I created this thread), and if I skip far into the "broken" file, it seems to be able to maintain a steady replay. But if I rewind into the first 5 minutes, I get the same buffer underruns in all the players.

I've looked into the CSC cache (Netcache) and the registry hacks to format the database, and I've tried enabling offline files, clicking "clean temporary offline files" etc and then turning it back off.. all in vain.

But to iterate: I can copy the entire file with a steady 80+ MB/s and play it flawlessly locally in all the players.

I know it all sounds like nonsense (well, it is) - but I can't for the love of god figure out why I can't stream that one file over a network.

Just found an old Asrock 330 HTPC which I hooked up to the ethernet - this slow Dual core atom with ION chipset played the file flawlessly across the network (actually it's connected to the router via an extra hub (switch)).

So it's this one file that's giving my I7 problems - but only when played over the network.