Old feeds appear in my reader

[aprmo]

Hi,
I've created an RSS feed (IMECA) that updates every hour.
When you go directly to the link the feed showed is correct BUT
if you suscribe using Google, the feed showed on your Google page is not updated! (it usually shows a 2 or 3 hours late feed)

Oddly, it will only update at 11 am, 2 pm and 4 pm (Mexico city time -0600).

I suspect a sever to be answering the request with a cached file but our server is not the one doing it.

Does anyone have an explanation for this?
Thanks