Resurrecting this thread to ask about a closely related set of symptoms: I have two Quadra 650's that each came with an internal CD drive (Sony CDU561-25), each of which has different problems.
Drive #1: about 80% of the time, it ejects the caddy immediately after putting it in. If it does keep the disc in, it reads it normally without errors, but sometimes spontaneously ejects it several minutes later (whether active or idle at the time).
Drive #2: when I got it, it would load the disc but generated frequent read errors and very slow data rates (probably retries). Now I just hear seeking noises while I get a spinning cursor, and after a few minutes I'll get a "disc unreadable" message.
My first thought was bad caps, and actually the boards are quite a mess of cap goo. I've recapped and thoroughly cleaned one board (not the other yet). But here's the funny part: both problems above follow the drive mechanisms, not the PCBs. Each drive will behave the same whether it's got the recapped board or the non-recapped one.
Earlier in this thread, techknight says that weak optics might cause the eject issue. I've cleaned the lens with alcohol on Drive #1. It still ejects discs 80% of the time but I haven't seen a spontaneous eject after the disc was loaded. I haven't tested Drive #2 again yet.
What else should I be checking? Are there other parts under the lens to clean, and if so, how do I get at them without damaging the mechanism?