Causes of CD player skipping?

Sony CDP-CX300 skipping - terrible skipping problem solved 

(modified post repeated for clarity)

I applied gun oil (a thin oil that says it has silicone in it) with a Q-tip to the single metal laser guide rail. That eliminated the skipping problem. Such a simple fix: remove the cover, dab a small amount of oil on the rail. Amazing that something that simple solved the skipping problem. I had observed that CDs always started skipping between 4.18 and 4:54 minutes. That was an indicator of a "location problem".

Detail: Remove the cover. Visually look for the metal guide rail (shaft). It is located at the rear left of the player (where the laser movement mechanisms are located) when looking from the front, and is seen in between the black plastic. The laser mechanism slides on it and this can be observed by powering the unit, selecting a disk, then increasing & decreasing track numbers on a rotating disk - the laser will slide on the rail. Now, reach into the access to the metal guide rail with a Q-tip moderately saturated with oil (3 in 1, gun oil in my case), and wipe along the accessible surface of the metal guide rail surface; the "backside" of the rail opposite the access area cannot be be reached for lubrication without further disassembly. But reach around "behind" the rail as far as practical. This lubrication task is very easy to accomplish and should have been included in the manual.

I had previously done some disassembly and cleaned the black "split drive rack"(gear), then re-lubed it with white lithium grease. [The pinion gear (not seen) and rack move the laser on the metal guide rail]. This lubrication did not solve the skipping problem.

These players are not sold any more, but it sure is convenient to load up as many as 300CDs - and avoid e-wasting an otherwise useful CD player.

So here's a weird one-
I cleaned and re greased the optical rails on my 200 CD Sony. Still skipped. As I was moving it, I lifted the back left end up (where the optical assy is located) which took the weight off the back left foot. It started playing fine, no skipping. I then Placed a spacer under the back side, just enough to take the back left foot off the tabletop. So far is playing fine.
Somehow the back foot of the case is tied in and stressing the optical assy to go out of alignment(?)
It's playing, so don't care at this point.

moolimbo,

Not so weird. I had a favourite CD player I had bought new in 1989 or 90 (Sony CDP-670) that started to skip in 2005 after a move. I ended up buying a really nice new Marantz to replace it. However, I kept the old Sony as it had a great Brown and Burr DAC in it and it sounded great otherwise.
Three years ago (2016), having moved a 2nd time, I decided to set up a 2nd stereo in my den and hauled out the old CD Player and thought ’why not try and fix it’. I went and read this excellent resource https://www.repairfaq.org/sam/cdfaq.htm and tried all the tips and tricks. I concluded after trying everything that either the laser was bad or there was a tiny crack(s) somewhere in the board. Because it had seen high use I gambled on it being the the laser and ordered a new high quality replacement and had a repair guy I trust put it in. He lived in my previous city so had to wait for a visit opportunity to get it fixed. I spent about $80 on the laser and repair. Then I drove the repaired player home (a 5 hour drive) plugged it in and arghh ... still skipping. So I put it away for 2 more years regretting I had even tried to fix it.
Last month with a little more time on my hands I hauled it out again and took it apart. I switched the cart out with one from a working thrift store Sony CDP-470 (lower model with no Brown and Burr DAC) ... but ... it still skipped. So I knew the problem had nothing to do with the cart/new laser. So I switched them back again. What struck me was that the skipping would happen on its own but also if I walked too fast by my CD Player (laminate on a basement floating floor)
So, I went back on line and read this thread. On a whim, after reading your post, I looked at the bottom of my CD player. Guess what? One of the little rubber 1/4 inch thick foot pads was missing from the back left side - probably knocked off during my move in 2005. So I took a pad from the CDP470, glued it on the 670 and ... yep .. you guessed it. CD player works like a champ. I have run it hard for 40 hours or so with no issues. I can also now walk beside it without causing skipping.
So my theory is that with the missing foot pad the plastic chassis was flexing up to a 1/4 inch which was enough to either 1) disrupt the movement of the optical assembly on the track/rails, or 2) flex the circuit board which may or may not have a crack in it I can’t see. The problem would have been made worse by having an EQ placed on it (added weight).
Since discovering this I see other posts complaining about the cheap feet on many CD players and the benefits of isolation products. So I have now ordered some sorbothane isolation circular discs. In the future I will also make sure my CD players are kept perfectly level, just like my turntable.
Moral to the story: Sony and many other CD makers used plastic cases/chassis. They bend. This can effect player performance so be sure this is not your problem before you take more drastic repair steps if your CD player starts to stutter, skip and jump backwards to track 1 (the symptoms I had).