accessing folder on server belonging to another domain

I have a VISTA Ultimate machine that belongs to Domain X where I have my account. I want to access (read/write) files on Domain Controller Y. Both machines are on the same segment on our LAN. Both the DC X that I belong to and DC Y are Server 2003 R2 machines. I seem to able to read and write files after a fresh boot of my machine but then things seem to go sour after an hour or so. My program can't write to DC Y. Any suggestions?