Anyone have the magic fix for a xp, 7 mixed network?

Have a mixed network of 7 and xp machines. The xp machines see the 7 machines fine. The 7 machines sometimes see the xp machines.

The problems is that when I first boot all machines the file and printer sharing works fine. But after a little while the xp machines are not allowed access to the files on the 7 machines. If I reboot the 7 machines it all works again. Any ideas?