much slower Activesync file transfers since 2 days ago

Hi,
I have a X51v and I can't figure out what I did on my PDA/PC that affected
my activesync file transfers. Now it must take about 45 min to transfer a
8Mo file!!
And weird enough, I figured that transfers are much faster (just like
before) while Activesync is actually syncing. It lasts 20s, and then it
become unbelievably slow until then next autosync.
I eventually have to keep pressing the Sync button while installing apps and
copying files... :-(

Has anyone had this problem before?

Thanks in advance!