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Tuesday 7 November 2017 at 15:35
My guess would be a cold solder joint that makes contact as the games warms up, but the odd pulsing behavior of U18 is not something I've seen before. Did you try replacing U18?
Source: http://pinside.com/pinball/forum/topic/bop-playfield-lamps-take-a-few-minutes-to-come-on#post-4061675