I'm back to share a trick that I learned about the Mossberg 500 series that includes 590 and 590a1. When trying to smooth the Mossberg action, I discovered that swapping out the elevator or shell lifter was the key. Luckily I had a couple in my parts bin. On one occasion I accomplished my aim by changing elevators from one gun to the other. I got the idea by replacing the trigger bar in Glocks to achieve better trigger pulls.
I'm providing a link to a place in Missouri that specializes in Mossberg parts and service in addition to other gun stuff. If you call, the owner will talk with you to figure out what you need. Parts are cheaper than other places, and shipping is most reasonable. havlinsales