Checksum error with Preupgrade

[marvin_ita]

Hi,
I'm trying to update from F10 to F11 using Preupgrade, but when it tries to download installer images I get

Fetched treeinfo from http://fedora.mirror.garr.it/mirrors.../os//.treeinfo
treeinfo timestamp: Wed Jun 3 00:15:53 2009
checksum failure downloading vmlinuz

I get this error every time I try. I was wondering how to force preupgrade to use another mirror, but I first compared checksums in some other mirror and found they are the same as the mirror I use.

Any suggestion? I don't get checksum error in normal file transfer, so I think this problem is not related to hardware or driver of my Ethernet card...

[marvin_ita]

Update: I've manually downloaded 'vmlinuz' from the mirror used by preupgrade and manually checked that the checksum doesn't check.
So I download 'vmlinuz' from another mirror and checked that in this case the checksum is good. So the problem is on files of the mirror used by my preupgrade, although in the txt file checskums are write good.

How can I force preupgrade to use another mirror?