Mediabrowser and NAS problem

I am trying to use a ReadyNAS Ultra to hold my media (mkv's) but when I select a file to play in MB there can be a delay of up to a minute before it will run. I have tried using a mkv file on a ReadyNAS NV and a Qnap TS101 with the same result. All the switches, NAS and HTPC are gigabit enabled and normal file transfer speeds are lightening quick. Disk spin down on the NAS is disabled. Media files on the HTPC HDD play instantly as do media files hosted on a networked PC. The ReadyNAS Ultra hosts all my FLAC music and the Squeezebox plays them flawlessly but there seems to be a big problem with MB and the NAS. I have tried optimising the settings on the Ultra with no improvement. Sometimes the file takes so long to load it locks MB or MPC-HC.

Is this level of performance to be expected from a NAS or am I doing something wrong?

NB: using MPC-HC + ffdshow + MadVR + Reclock

Update: VLC media player starts playing immediately from the NAS and the WMC native player does exactly the same. Seems the problem is with MPC-HC and/or the video or audio renderers. Running MPC-HC directly (rather than via MB) to the NAS gets a movie playing in about 12 seconds but if you stop it and start another movie it takes 60 seconds before it starts playing. Same behaviour as via MB. Same movies play immediately in VLC.

I wonder if I should do a fresh install of MPC-HC and the codecs?

LAV splitter is being used.

Thanks. I saw that but am using UNC anyway and the NAS is showing as connected. There seems to be something in MPC-HT causing the problem but no idea what.