Problems streaming from iMac to Apple TV 2 (but not internet)

I am able to stream rented content from the internet fine, however, I encounter problems streaming content from my iMac via my Airport Express (both iMac and Airport Express purchased 2007). Videos from my iMac will sometimes start streaming (but not always) but will often get 'stuck'. There is also a large lag time between videos (e.g. if watching a selection of music videos). Sometimes streaming will resume, but more often it will not. The same problem applies to accessing photos from my iMac.

After conversations with staff in the Apple Store, I tried a number of things such as using different channels on the Aiport Express. I also moved my computer to be line of sight with no walls (previously there were 2 walls).

I think that the problem is related to iTunes or the Airport Express as I have recently experienced the same issue streaming music from iTunes to the Airport Express station that's connected to my music system (either cuts out or stops completely - when I tried to reconnect the speaker, an error message appears).

These problems surfaced after moving house so the only other things I can think might have caused this are the Airport Express' positioning vis-a-vis other electrical equipment that could cause interference (although this seems unlikely given I have the same equipment) or maybe connected to an upgrade of iTunes (I read somewhere that someone had problems after going to version 10).

Any insight in to the matter would be much appreciated.