Guild Wars 2

I have this very simple script that requests the same api page (/guild/membership) using the access token as a get parameter. This work perfectly except that once in a while (about 2-4 times per week) I get HTTP Error 400: Bad Request. 10m later when it reruns everything is general works again with the identical request, which implies that the Error should not be 400 but rather 503: Service Unavailable.