If the problem started when you replaced the chain and cassette your problem may be with the chain or cassette. I'm not much of a mechanic but job requires regular trouble shooting. Usually if a system is working and then you change part of the system, and the system stops working, you go back to the part you changed to find the problem. Example "I bought new shoes, now my feet hurt, so is something wrong with my feet? Or are the shoes too small?"