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.