Over time you can get micro fractures and corrosion in the solder joints at the terminal. This can reduce the current carrying capacity of the coil and hence its "power". Check to make sure the lead wires from the coil haven't broken loose and are just laying on the solder instead of being embedded in it. Also, in older coils, make sure the EOS surfaces are flat and clean. Cupping or burrs will reduce the surface area available for carrying current and will reduce the "power".

I know this was an electrical discussion, but don't discount the effects of friction inside the coil sleeve. Both from the sleeve melting/warping and the armature mushrooming. I suspect that there is an inductive heating component as well on the armature, especially in coils that get a lot or rapid repetitive power cycles, like flippers and pop bumpers.