PS2 slim skipping CD-audio

Ok se here's a boring issue.

I've got a PS2 slim (77004) which I've repaired (cleaning, new laser, new flex cable, and then some, definitly spent more on this thing than I should have). After thorough testing it seems to work perfectly, except... On a pretty scratched Wipeout 3, some of the music tracks (CD-audio) is skipping. Since it's only some tracks, I assume it's because they in particular have scratches on them, but they play fine in another 77004 and a 50004 so the disc isn't TOO bad so it seems this laser is in worse shape than in the other machines.
The music only skips when played within the game - when playing it as an audio CD in the PS2 browser, they play fine (maybe it's read at a faster speed and has more time to error correct there?)
If I set a problem track on repeat in the in-game menu for a while sometimes it's no problem at all, sometimes it skips really bad, and everything in between.
Setting double disc speed in the PS Driver does not help.
The skips are real skips, as in jumping around a bit. Never silence or so.
CDs are detected very fast and no problems with FMVs.
The laser is some replacement from ebay, I don't remember the details I'm afraid but I think it was (supposedly) new and original.

Does anyne have any kind of advice? Could this be caused by something else being broken (spindle motor, wormgear, etc)? Could the laser be dirty? Would it help to adjust the CD pot? (Would that give this problem?) I'm thinking decreasing the voltage, see if the problem gets worse, if it does, increase it until it goes a way. However I know the risks about this (especially since the laser seems so close to working perfectly as it is).

I do have the old, replaced laser left, it is broken so that it have a ~30% chance of disc read error (the disc spins but it doesn't seem to even try to read the disc). I suspect it's because of dirt in it the flex cable connector which I can clearly see but is hard to reach and clean without breaking it... the flex cable is replaced so is clean of course. But the problem might as well be caused by dirt within the laser assembly as well (previous owner seems to have dropped plenty of some unidentified liquid all over the machine, some even on the mainboard!)

I'll probably start with putting back the old laser, and see if it reads this scratched game better - then I'll at least definitly know it's the new laser that's bad, if so I guess I'll try to clean the old laser properly and see if the problem with that one goes away. (Less risky to try to clean the old one than to adjust the new one, me thinks).

Any ideas? Sorry for rambling post, rambling replies are appreaciated