That's a long time for any well to last. Especially one with a screen.
If it only did 5 gpm when originally installed, I would assume that the screen was a very fine mesh. Like 100 gauze or so. This would have been done because of the sand size. You want to be sure to filter it out once the well has been installed. Problem is, you don't know this for sure. So unless you pull the old one and inspect the sand stuck to the screen and try to determine what screen size to use for the new well, you may always have a sand problem. You can also, pull the old one, install a new screen and put it in the same hole. It will again pump water if put back where it was.
Edit: I just saw your other post. I wouldn't call those guys again unless they will take the pump back. The pump didn't cave in the well. That isn't possible, but if those guys turned the casing, they could have torn the screen. If you don't have the ability to pull the well, try to find someone who can or look for a new drilled well out in the yard somewhere by a pro.