PAL Dreamcast (not only) audio issues

I bought this PAL Dreamcast last summer, for 50€ (about 68$), with two controllers and a VMU. I thought it was a nice deal, but in fact this system revealed to have a really weird behavior.
I had no games, so I borrowed Shenmue and Power Stone from a friend, made backups and played 'em. Power Stone worked perfectly; both the original and the backup never froze, never had audio issues or whatsoever. The Shenmue backup CDs, on the contrary, had serious pitch problems in most of the tracks, absolutely no sound during the scene in which Ryo drives Nozomi home on the motorbike (remember that somewhat cheesy song?), but the dub was otherwise perfect. It's kinda difficult to explain these "pitch problems", try to imagine the music that plays when reading the notebook suddenly shifting pitch up or down when changing pages. I managed to beat the game, technically it was absolutely beatable... but the audio was SO annoying at times.
I thought it was just a backup problem, that maybe I messed up something when burning the CDs. From what I learned, CD-Rs tend to stress the laser lens, so I believed that it made sense, after all: Power Stone is a light game, Shenmue is not. And since the friend of mine was kind enough to let me borrow those games with the promise of paying attention to pay the max attention in order to not damage them, especially Shenmue, I didn't try the original Shenmue at all.
I also noticed that the third disc seemed not to work, with a quick look online I was able to recalibrate the lens. Sadly, this didn't fix any of the audio issues.

2-3 months pased, and then I found a SUPER deal, a guy in town sold me Chu Chu Rocket, Sonic Adventure, Crazy Taxi, Phantasy Star Online, Resident Evil 2 and Skies of Arcadia for 30€ (about 41$). All of them seemed to work, but only recently, when I started playing them for 1-2 hours, some audio problems surfaced again.
I then recalled that friend of mine, I wanted to make a random test: we took his Moero! Justice Gakuen original disc and a PAL Project Justice backup (it's the same game, missing some party and custom content). The Japanese one worked flawlessly, the Project Justice backup had audio issues AND often stucks on the loading screen before a match. It doesn't froze, it just loads endlessly.

Let me make a summary of the behavior of this machine:

Shenmue PAL - I don't know.
Shenmue PAL backup - Suddenly pitch shifts during almost every track, no sound during the Ryo and Nozomi scene. The dub shows no problem.
Power Stone PAL - Perfect.
Power Stone PAL backup - Perfect.
Chu Chu Rocket PAL - Perfect.
Crazy Taxi PAL - After about one hour of playing, the music becomes weird. Like one of the two channels, left o right, has an annoying little delay. In truth, this problem shows even when the game is set on Mono. It's just an example of how... out-of-synchro it feels.
Sonic Adventure PAL - Same Crazy Taxi issue, but often, during particular "heavy" scenes, an annoying crackle shows up.
Phantasy Star Online, Skies of Arcadia, Resident Evil 2 PAL (all of them) - I didn't get the occasion to play these games extensively.
Moero! Justice Gakuen JAP - It's ok. I think.
Project Justice PAL backup - It sucks. A lot.

I am 100% sure it's not an audio output problem, since I got these results from both my old TV & my PC monitor, using my PC speakers.

1. I initially thought it was a laser calibration problem. It doesn't seem to be the case.
2. The Crazy Taxi and Sonic Adventure behavior made me think that it could be an overheating problem, since these problems start after an hour of playing... but I'm not sure, from what I read, overheating manifests into sudden freezes. No game ever froze.
3. Do I have to clean the laser lens better? Last time I used a common cotton stick with the common alcohol I found at home.
4. Do I have to entirely replace the laser lens?

Thanks for your attention! I would be grateful for a hand of help.

 

i had some werid faults way back but was covered for 4 years when i had the console back in 1999.
audio faults i have a spare motherboard here i would do a swap as i like finding fault's if you want.

 

I forgot to mention that I'm using a Dreamcast SCART cable, and right now I have no possibility to try another cable. :/ EDIT: I don't think it's a cable problem, anyway...

I'll take that in consideration! It would be very kind of you, but I would like to read some other replies before.