I had a Time Machine many years ago, and this problem. Too strong flippers.
I got it so that I connected (power /5W) resistors on additional boards mounted in the cabinet below the flipper coil switches (in series with a contact)
It was many years ago, but if I remember correctly resistors value was 1 Ohm (remember, MUST be a wattage ceramic resistor)
Feel free to try with values between 1 to 2.2 Ohm....choice depends on yours "taste" : ))
Greetings from Croatia