I just bought a LD player, pioneer HF9G
All is clear, and I saw some movies.
Just yesterday, I watched cowboy bebop episode 2, and during the last 10 min, the player makes strange sounds, like klik klik klik ....
The disc is super clean, and it always happen from the 40th minute of the disc. No problem at screen, the image is clear.

Disc warped?

doesn't seems to, but maybe it is the problem ?
How can I be sure to have a disc perfectly flat ?

simply put it on a flat clean surface and look for bended disc. It should touch flat everywhere!
Only had one disc making a noise nearing the end of one side. turned out it was slightly warped.
You should be able to flat a disc by putting it between a stack of books or something similar.

Good luck

ok ! I will try that !
How many days are good ?