I needed to fix my original PS1 with the same issue... I am not an expert at repairing, but in my experience, it seems to be most commonly caused by three things.
Disclaimer: I have almost no technical skill, and my "solutions" are hackish, and something that a trained-monkey could achieve
1) Heat
The PS1, seems to have an overheating issue, and when mine started doing it, I had to place ice-packs on the bottom of the system to correct it. -- Be careful of moisture if you do this, as the ice will melt. (Yes I learned this the hard way).
2) The Laser
You already checked this.
3) The lid
My original PS1, a top-loader as most are, had a faulty lid, that would ever so slightly pop-up after clicking shut. This movement, while quite subtle, would somehow be just enough to interfere with discs being read, but it would not be enough to trigger the lid sensor, so the system would never complain about the lid being open. I "fixed" mine by placing a heavy rock on the lid while playing a game.
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If none of those seems to be your issue, I would then start to question the discs themselves. Can the system read regular Audio CD's?
Beyond these things, you would need someone more experienced than I am in this area.