Problem with installing new laser

[sparc]

Because my V5 PS2 had reading issues ever since I dropped it 2 years ago I replaced the laser with a new C model laser (used lenschanger). The laser seems to work fine. During callibrating the laser (using both screws) I could never get it right. Booting of games is no issue but there seems to be a problem with reading the last 15% of a full 4.3gb DVD. I'm using HD Loader to test how well the PS2 is able to read discs since it has a nice speed-reado-meter. No matter what I try I can get it to read at 'full' speed past the 85-88% of a full written DVD (Xenosaga 2 disc 1 and DMC). After that it always drops to around 350 kb/sec (and the speed with which the disc turns drops). Basically this happens nearly instantly when the read speed reaches 1600 kb/sec with all discs I've tried. My guess is that the drive motor isn't correctly alligned (perhaps because of the drop 2 years ago) and thus the distance from the disc to the laser differs (innerring versus outer ring). But that's only a guess, trying to correct this doesn't solve the issue. I'm really starting to wonder whether my old laser had really gone bad.

I've tried quality media, older Verbatims and newer 8 speed Taiyo Yuden DVD+Rs (burned at different speeds).

Anyone help on this issue would be appreciated since I'm slowing getting insane after spending two days to align the laser correctly.