I have a Saturn, but it doesn't work properly.

We get the splash screen and can get to the system menu. It runs audio CDs fine.

I've tried it with two games: Sonic Jam and Nights.

Sonic Jam doesn't work at all - it just plays a jarring noise (from the TV - not the console). Video: https://www.dropbox.com/s/0w67ohjer5..._7788.MOV?dl=0

Nights runs, but incredibly slowly. I also wonder if the disc is running at full speed in the console? Could be my imagination but it looks a little slow to me. Video: https://www.dropbox.com/s/rlzurhnr1t..._7787.MOV?dl=0

I've given the lens a clean with a Q-tip and rubbing alcohol but to no improvement. I removed the case and nothing looks obviously wrong or broken.
Tested the PSU with a multimeter - it seems to be sending the right power. My buddy tried adjusting the lens screw just for the hell of it but no improvement. I think there's a chance he made it worse.

My friend runs a used game store. He had a spare Saturn in the back. Both this Saturn and mine are model 2s (round buttons) but have different-looking PSUs. As an experiment we put the laser unit from the spare Saturn in my Saturn. It results in... EXACTLY the same problems, with both games.

Any suggestions?