It's normal for games of this era to momentarily dim the lighting as the flipper pull. You can do some things to minimalize it.

*Check the transformer jumpering and make sure it's jumpered to match your regions line voltage. If the game is jumpered for high line voltage, (120v) but your line voltage is 115v then the transformer output will be a bit lower.
*check the adjustment of your flipper EOS switches by pushing in the link plunger with your finger. The lower flippers have two switches and should be adjusted break-make. The inner EOS should open at about 80% of the stroke and the outer upper activation switch should close an instant after.
*Rebuild the flippers with new EOSs and coil sleeves.

That's the best you can do. You will probably have to live with a small amount of dimming. In the factory we used to test games by continuously flipping both flippers at the same time and making sure the game did not reboot or the dimming was too noticeable.