Quoted from Markharris2000:

I know I am late to this thread, but I am very curious what happens mechanically that makes flipper fade and fans important. I know the there are countless enthusiasts that experience the effect of overheated flipper coils, and sluggish flippers that result, but I am genuinely curious WHY heat changes the operation of these purely mechanical mechanisms. What does the heat do to the mechanism to make it sluggish?
Just a guess, but does the coil sleeve contract around the plunger as the temperature goes up and everything expands? Would brass coil sleeves solve the problem without fans?

Quoted from wisefwumyogwave:

Say what? When things heat up they expand.. so when the coil wire starts expanding from heat (due to the friction of electrons moving thru the wire in the first place) free electrons then have to travel a further distance to the next copper molecule, this is measured as increased resistance in ohms.
Your voltage stays the same and with increased resistance due to heat, you end up with less amperage running thru the coil so less wattage overall, which decreases the corresponding magnetic field strength.