you said the chain really started having problems after you changed your back tube??

Forgive the duh question but are you sure your back tire is fully seated?? I was having major problems with shifting one day last winter - could NOT get the chain to shift or when it did it kept jumping the cogs. When I got home and looked at the rear cassette I noticed my back tire wasn't seated on the hub. Oops! Once I reseated the tire, my chain issues went away.