Take one blank media from same pack you're using to burn games.
Burn a region unlocked DVD movie.
Remove the PS2 mod.
Try the movie in it.
If it works then the mod is wrong for your PS2 or is causing the reduction of the reading capability of the laser (It may be close to be bad so the chip might make it read worse). It seems to be fine install wise as the unit still play fine the originals.
You can also try the replacement laser as shepd said.
Hm ... usually a PS2 with this kind of weird behavior has some kinky mobo problem like cold soldering on the DSP chip or other things as a broken resistor ... Man, some of these defects can be hairy-scary... Really pretty hard to find even for a experienced technician.
Good luck with it
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)