I am unable to reproduce the error as of right now. My feed depends on an external system, so it could be that that was the problem. Sorry for the confusion.
respect http cache/expiry headers for setting next sync date
(a prerequisite would be to have per-collection sync intervals.)
Then you could respect http header meta-information, e.g. when it tells you how long this resource may be cached or when its expiring (and then trigger a new sync).
This could help save bandwith and computing time on the long run if servers send correct headers.