PlayStation 2 Won't Read Discs Despite Replacing Laser

I got in a PS2 30001 model and have cleaned and tested it. It runs Free McBoot and OPL decently. But when I try to run a DVD or a game, the thing craps out. I've opened it and checked if the disc drive is working as it should. Ribbon cable isn't torn. Laser assembly runs and lights up. I've replaced it with two other C series lasers, but still nothing at all, even after checking the pots and "solder points." It knows it's reading a disc, but doesn't do anything after that. Has anybody encountered this?