This occurs when the laser optics can't track or read the data on the disk. Most people would probably not notice this problem until they have fitted a chip and tried using golds, this is not a problem with the chip but the same problem I have just described originals should work the same with or wthout a chip fitted. The problem is more apparent in earlier versions of the Playstation, Sony tooks steps to combat this problem in later revisions of the machine. If you have an early PSX you can see on the follwing zoomed picture a PSX laser that has a worn sled unit that needs repair, as you can see one side of the laser is sitting lower than the other, this causes focusing problems which cause the skipping. If your machine tends to play better when you turn it upside then it is a sure candidate for lazer wear and adjustment.
Adjustment usually provides good results on the later machines that have the alloy sled.
This control sets the default DC voltage flowing through the servo coils in the laser that move the laser lense up and down, basically it sets the default distance between the surface of the disk and the lense. This adjustment is the one that has most effect when adjusting to cure problems. This control sets the voltage that is added or subtracted from the bias voltage by the servo circuits for a given fluctuation. So when you have a slightly warped disk the lense can bob up and down and follow the surface accurately, if this setting is not correct the laser will not accurately track warped disks. Adjusting the bias control slightly clockwise will increase the bias voltage and therfore move the laser slighly closer to the disk, this is usually sufficient to cure the problem for most machines.