Your uneven firing can be caused by a failed power relay, about $60, or one or more failed heating elements a set costs us about $200. By the time you notice a difference in witness cones, or your electronic controller errs out, you've lost more than one heating element.
I'm a big fan of the "paper test" where you place paper strips under each element to find strips of paper which don't char within the first few minutes after firing the kiln on a fast speed. It's often more accurate than watching for glowing wires but provides somewhat less information than ohmmeters. A monthly paper test will tell you about failed elements long before witness cones do.
If all of the heating elements controlled by one relay are not heating, then the relay needs to be replaced. If the failed elements are not all of the elements controlled by a relay, the failure is merely those heating elements.
After running an 18 month experiment at our studio replacing coils and relays one at a time, we've resumed replacing all wires and relays at the same time, about once a year for our heavy use.
We discovered with our particular kiln that after one element fails, it's not too many more firings until almost all of them fail. Additionally we learned that the specs on the relays are designed to last longer than the heating elements, but not much longer. Know that relays can occasionally fail in an on position.
For us it's economical to replace the heating elements and relays about once a year rather than deal with frequent firing failures. An exception would be a single element which we know has failed prematurely because of mechanical damage, or damage from splattered glaze.
If its a potter brumfield relay (the black ones in most skutt & L&L top loaders--not a mercury relay), even from skutt or l&l theyre ~30.00 a piece, order them from allied electronics or comparable and theyre under 15.00 a piece.