Intermittent 404 Error from Javascript/Jquery/Jsonp API call

Hi

Have a webpage:
http://www.walkingclub.org.uk/this_week.shtml

It uses Javascript/JQuery/Jsonp to call the blogger API - example URL:  
http://this-weeks-walks.blogspot.com/feeds/comments/default/?alt=json-in-script&callback=jQuery110208666548756882548_1379263597217&max-results=50

You can go to this URL to see what should happen

It has been happily working for years, but recently, has started to fail 
intermittently (say 1 in 10 times), and returns a 404 error instead

Is something broken?

It could be I changed something which caused a side effect.
1) The www.walkingclub.org.uk pages are served via Google Pagespeed 
Service, but going directly to our server makes no difference (remove the 
www. ) 
2) I tried delaying the failing call (#3 of 3) with a setTimeout(...,2500) 
- made no difference
3) Its the "blog comments" API call that fails - the "blog posts" API call 
seems fine.

If I go to the Feed URL above, and refresh the page several times in a row, 
I can get the error.

"The feed specified by 
http://www.blogger.com/feeds/13665817/comments/default is not enabled"

The response headers are:

HTTP/1.1 404 Not Found
Date: Sun, 15 Sep 2013 16:57:07 GMT
Content-Type: text/html; charset=UTF-8
Server: GSE
Expires: Sun, 15 Sep 2013 16:57:08 GMT
Cache-Control: public, must-revalidate, proxy-revalidate, max-age=1
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
ETag: [not shown]
Content-Encoding: gzip
X-Frame-Options: SAMEORIGIN
Content-Length: 103
Alternate-Protocol: 80:quic

Thanks for looking at this

Andrew

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