Have not heard this issue before for Stern games, and normally only an issue on WPC games with a dirty flipper opto.
The extra load of the display will cause a small drop in the 12V unregulated supply output. Normally not an issue, but if a flipper opto is dirty and already operating near threshold, the small voltage drop can result in a noticeable difference. Cleaning the opto resolves the issue.
On a Stern game, this is unusual. Does unplugging the display increase the strength of the flippers? If you measure the 12V unregulated supply with a DMM, do you see a significant difference in voltage when the display is connected/disconnected?