Hi @LeeB,
I wanted to comment that to resolve this issue for us I set the WCF throttle or minFreeMemoryPercentageToActivateService in our web.config file equal to “1” or 1% of memory before it would not allow new service calls. I believe the default is 5% and you can also set this to 0 which means no throttle. Here is where I got the information and suggestion to do this:
Since setting this, we have not run into any further “…requested service…blah blah blah…could not be activated…” issues.
It was a bear 
(<–it wasn’t that cute) trying to figure out exactly where to put this setting so I hope it’s okay to post this to help out.
PLEASE be careful making this change because, someone correct me if I’m wrong, but any changes to the web.config file might auto-trigger some IIS\AppServer resets or something to that effect…I think? I don’t make changes LIVE so I wouldn’t know. 
(cough) And I would definitely test this in a TEST environment first and make sure everything still runs correctly and then try making the change during a maintenance window and see how it works for you.
web.config file:
Link to entire post - E10 Memory Issues: