First: how is Cowtown? Still snow on the ground? Too bad about the Stampede.
Anyway, I assume you replaced your chain. Chains are 3,000 mile (max.) throwaway consumables, and you may be experiencing a new cassette not playing well with an old chain. The old chain is probably too short - that is if it was sized correctly with the old 28-tooth cassette. Expect that when you shift into the big cog at the front, and the largest cog at the back that this will tear off your rear derailleur, and carry the fragments into the front derailleur. Which will destroy that and perhaps a couple of the chainrings as well.