This is a very common failure on the grey PS1. This is either caused by a all plastic laser, or a faulty optical block. The die cast ones fail from optical block failure and the plastic ones wear unevenly. Adjustment can help, but it doesn't last forever and it's not a super reliable way to fix the problem. It works, but it's only recommended if you have nothing left to lose.
My recommendation is to get a SCPH-10x unit. It isn't prone to this problem, but the Parallel/Serial ports are no longer on these units. Most people do not need them, but if you do get one of the early 700x models.
If you choose to adjust it, there's a point on the motherboard and the laser assembly. You will need an oscilloscope to adjust the board trimpot, and for the laser trimpot do it in small doses until you get a near 100% hit rate.
Messing with the board one is NOT RECOMMENDED. You do not have to adjust it so leave it alone. You want to adjust the one on the laser. It isn't as effective but it's a lot safer to adjust then the motherboard point is.
Now if you are brave and are willing to risk it, mess with the one on the motherboard. You will risk burning your games but it does work a lot better then the one on the laser assembly. You can follow this guide on how to adjust the laser, which explains how to do it.
PlayStation Optical Drive Replacement