Hi Stan, I'm sure others with more technical know how than me will chime in here but for me whenever I've had intermediate flipper issues like the ones your describing its because something has either come loose or is binding. The coils are unlikely to be the cause because they either work or they don't but you should take a look at the flipper coil sleeve and ideally replace or clean it if it's dirty (you'll know if it's dirty, it'll be black). Next I'd check the rest of the flipper assembly and make sure everything is tight. For example If the bracket that the flipper is mounted on on the underside of the playfield is loose it can drop and cause the flipper to slow. I'm not familiar with the particular flippers on the game you have but there will also be a plunger which needs to operate smoothly so if there is an associated bracket loose this will also slow things down.
Good luck with it.
+1
When the coil gets lazy, I'll bet it is also getting hot. This is a sign the EOS (End of Stroke) switch is not gapped correctly, or a wire has dropped off. Look at the other flipper EOS and with the game OFF, attempt to adjust the faulty flipper assy. An annoying trend of late is finding the Fliptronic board over fused. If the coil is playing up, ut will eventually blow the fuse, and if the fuse is over rated, the next weakest link in the chain will be the driver transistor which will require a board repair. Could you check the fuses in the fliptronic board are all 3 Amp Slo Blo ?
