As Ben has already mentioned (
Make sure the chain is clean and properly lubed and you dont do it too tight) IMO it does appear that you could be adjusting it too tight, which would explain why it keeps stretching. If you have any tight spots in the chain make sure you adjust it by taking the reading at the tightest spot, not the slackest. I personally find it stretches less by running it
slightly on the slack side
According to the Haynes manual:-
Drive chain slack to be between 30mm and 40mm
What I've mentioned I realise you may already be well aware of
One other possibility is worn out cush drive rubbers
Chris.