Where did you get the leds from?
The cointaker ones normally don't flicker, but I have seen some cheap Chinese ones flicker, you may have one of these in there.
Are you getting the right voltage at the bulb?
The game came already with the LEDs, so I'm not certain where they came from. Also, no problem with the voltage at the bulb.
Sounds like the previous owner has installed cheap leds then, got to
http://www.cointaker.com/ their prices are great and so are the leds.
I normally get wide angle warm-white and although not quite as yellow as filament bulbs they are not blue like some leds.
I have used them on GI and feature lights, on an old Bally or Stern they will take some load off the GI circuit.
