I had been running Kernel 4.2 on my main system since it was released, but have been experiencing vague and inconsistent problems with sluggish applications, which sometimes hung for about half-a-second.. Nothing specific, and no apparent errors reported..
I reverted to Kernel 4.1.7, and everything seems to be back to normal.. I will try 4.2.1 as soon as it is available..
Robert Gadsdon. September 14, 2015.