MS SQL identity value being reset

We have multiple MS SQL 6.5 servers running and our issue is only occuring on one of the servers. When we reboot the server (or, I believe, when we stop/start the server services) the value for our IDENTITY classified fields gets reset to a previously used value and causes some duplication of record id's or errors where the UNIQUE classification is used.

Any suggestions as to a cause and or solution?

Thanks in advance for your help.