Admin Client Issue

We have an issue that just started happening recently. If you attempt to open a mail file from the Domino administrator, it takes a looooong time to open; however, if you go to the Notes client and select File > Database > Open and select the same file, it opens up immediately. This has happened 2x - we reboot the server and everything goes back to normal for about 2 weeks. Any help is very much appreciated. Thanks.