Ever since I had to replace he 72 pin on my NES due to a bent pin on the original, it has never wanted to play games. It's very finicky and I always have to wrestle with it. Then, 20 minutes into a game, it'll freeze randomly.
I read that some of these aftermarket slots are too tight, and another user posted how one of his NES unit only works with the game in the up position! That got me thinking....
So I took apart my NES and plugged it into a TV. With a handful of carts I've been having trouble with lately, I experiment with holding the cartridge slot in various up positions. Lo and behold, about a 1/4 of the way pressed down, all the games fired up on the first try.
I removed the springs from the cart slot mechanism, and cut two short sections of a bic pen shaft, to replace the springs and force the cartridge tray to remain in the up position that I found worked best. I reassembled the NES, and now games work perfect every time. It's quite a simple solution and may work for you!
Cartridge tray resting on the pen shaft
Lifting up the cartidge tray
Looking into the assembled NES, you can see the metal cross bar sitting lower, but carts slide right in.