Pre Hyperglide models used 6-speed Uniglide cassettes which are hard to find, my advice would be to upgrade to Hyperglide which are more frequently available but you will need to change your freehub to Hyperglide too. Most 6-speed thumbies will acomodate the extra gear. I'm guessing your bike is Exage Trial.
A useful source of info is http://www.sheldonbrown.com/k7.html
If I were you I'd first try a new chain to see if that fixes the issue, if not then you need to establish whether slipping occurs on the front crank rings or actually on the rear cassette, if the rear then you deffo need a new cassette.