So there are a ton of LED problem type threads but I tried this yesterday on the advice of another pinsider and was really shocked as to how well it works.
For me, this is mostly an issue on early Solid State Bally/Stern games but it does also apply to other era machines...
I often have a game where I only want to put a few LEDs in it in order to brighten certain things up. Like replacing an incandescent GI bulb or a purple/green feature lamp simply because it is not clearly lit.
If you use LEDs that have an unwanted flickering effect, simply solder a 1.5K 1/4 watt resistor across the two bulb lugs and all ghosting/flickering is eliminated.
This would obviously not work well for an entire machine/playfield but is great if you are only adding a few LEDs here and there that have unwanted flickering.
Total cost is like .05 cents for a resistor. If you have already purchased LEDs that ghost, this could help out some on certain projects.
I do realize there are aftermarket PCBs and anti-flickering LED bulbs now but if you already have purchased a bunch of LEDs that have this issue, it may help someone.