Could be the media and/or burning method. Some media doesn't burn well at 4X with some firmware/drive combinations. Some PS2's won't read 4X burned discs well regardless of this issue. It is usually best to go 2X if it isn't a mainstream mfg. like Pioneer, Sony, Verbatim, TDK, Ritek, etc.
Other than that, may be your v4 or chip install. Hard to say for sure with out more info on the Hardware/Firmware/Software involved.
If it ain't broke don't try to fix it! But, hell if you can tweak that bastard just a little bit more....hehehe.