I concur it might be time for a new cassette. I use KMC chains all the time on SRAM and Shimano cassettes but I always check the old chain first to see if it is stretched before swapping. If your cassette is worn you can usually tell by putting the chain on the smaller cogs in the rear and pushing hard on the pedals (say going up hill) to test whether it skips. Be prepared for the chain to skip so you don't get caught off balance.