As you clean and dust each coil, be careful not to jiggle or move any of the copper straps that tap the coils. You might grab each tap to einsure they are tight, but do not change their location. If you have a permanent marker, place a mark on either side of the coil clip, so you'll know where it belongs should it fall off. If rotary coils are used, inspect the wheel that rotates around the coil ribbon or tube. Age and arcing can damage or destroy the ribbon or tube.
Failure can be disastrous, so look for pitting or burn marks.
All connections into and out of the coil should be snug, but do not over- tighten. Corrosion on connections can cause intermittent problems. Your time is better spent replacing the corroded hardware than searching for intermittent faults. Steel hardware is affected by the RF frequency. Ferrous hardware will drive you crazy trying as you try to track down what's wrong.