Problem converting D800 NEFs to Dngs with adobe converter

What's happening is that the files begin to convert one every 7/8 seconds but gradually the process slows until it's taking over a minute for each file.

I end up having to do a re-start as the PC eventually freezes.

Incidentally the PC is one built for me by a colleague, not overly high spec but with an I3 processor/8 gigs RAM./ 2TB hard drive

Loads of suggestions from it's a problem with the Windows installation to an underpowered power supply (it's a basic 350 watt p/s).

I had the same problem with transferring from the memory card until I bought a usb 3 reader.

As it happens I can work from the NEFs ok as I now have Photoshop CS5 but I would like the option of dngs too.

Anyone any ideas please? Does anyone else experience this problem? Thank you.