Varying bit rates

What are you using to measure the bit rates? FLAC when streamed is variable but down to 130kbps does sound extreme.. Was it pure silence? 

As far as the double tracks, artwork etc, these do sound UPnP media server app issues. Other than trying to rebuild the database my advice would be to use either MinimServer or Asset instead. These are highly regarded and tend to work well without much grief

Twonky is not highly regarded, and a decent server is the way forward I feel. That may well mean getting a new nas, unless you run Asset or Minim on a Raspberry Pi. 

Thanks. Others have been less critical of Twonky on this forum, so I'd be interested in other views too.

I did have problems when I used Twonky but not those you are describing with bit rates.  I did have on going problems with double tracks appearing, or tracks and albums disappearing and with tagging and album art - from memory this was aggravated when I put new music on and it was updating the database.  Moving to Asset resolved all those problems.    

I don't think it is easy or even possible to install Asset or Minimserver on my Buffalo Linkstation. Please note I have no great technical know-how.

Google is your friend for this Steve as I'm sure there will be others with the same NAS who have had similar problems.  WIth my first NAS it was not possible to move away from Twonky and, eventually, I bought a different NAS and used the original as back up for my files.  Hopefully there will be a solution out there for yours

Yes, I've been googling but not with a great deal of success so far. To be honest the issues are an annoyance more than anything else, but it would be good if the set-up worked better. It seems that with this NAS drive having Twonky built in it may be difficult to make progress without changing Nas drive. I'm reluctant to do that at the moment. I presume that if I did I could just store all my saved files from dbpoweramp in a new media server/Nas drive.

Yes, you just copy your files to the new NAS, or if you have a PC, you could put Asset on your PC and copy the files to the PC or to an external drive connected to the PC.  I tested Asset on my PC and was very happy with it, ran that for a while with the only downside being that I had to have my PC on upstairs to steam music.  After a while I bought an Asset NAS which has been rock solid but there are plenty of good NAS drive around which will run Asset for decent money. 

One other thing which occurs to me is that my wife's itunes files are also stored on the Nasdrive in a separate folder. Could these be getting mixed up with the files I ripped with dbpoweramp?

I don't know that NAS but within Twonky you should be able to select which folder it uses to build its database.  Worth checking that it is only tracking the folder you want it to.

I think the IT guy who set it up for me set up the media server appropriately.

I managed to disable my wife's itunes from being picked up. Then I did a rescan in Twonky but this has created more duplicate tracks! 

So, next I reset the server and cleared the cache. This seems to have removed nearly all the duplicates but there are still some left. 

It sounds like you've at least made some progress Steve.  I had similar problems with my first NAS and Townky and in the end I gave up on it and moved on.