The router is a netgear dg834N and is a new repacement only just over a month old I will try and get one of the snap choke and put it on the power lead to the desktop. Is the snap choke the same as the ferrite collar on the monitor cable if so themonitor cable has one.
I have been advised it might be rein but finding it difficult to detect if it is.
I have left the desktop switched on all night with the monitor switched off and the SNR did not drop enough to cause the synch to drop off, however during the day the SNR has dropped periodically and switching the monito off for a minute or two results in the SNR increasing it has now remained stable at about 6.5. So the monitor does seem to have some effect as by switching it off as SNR drops causes the SNR to increase.
If anyone has any further ideas I would be grateful