Thanks for the replies! It sounds like proper wheel building is not a task for the novice, though it sounds like I should invest in a spoke tension meter.
Iride, I have never had a problem with the chain jumping off of the cassette. In the last year or two I have started riding off pavement more and more. I am not doing mountain bike stuff, but some the gravel / grass trails are certainly a bumpier than pavement. That, and me being a bit of a clydesdale probably have something to do with it.
It sounds like I need to get a good rebuild on the wheel, then learn how to check the tension and make necessary adjustments.
Thanks!