Worse disc read performance over time - what am I missing???

[Spleet]

Sounds like a PSU issue to me, try and get one of those Pico's or a DreamPSU

I second this. When I would use exclusively discs to play, I would experience crash to dreamcast menu more and more frequently. Getting a different power supply eliminated this issue. It was especially prevalent wit Quake3 Arena

[John Travoltage]

As suggested, I've taken a punt on a PicoPSU, which just arrived today - unfortunately, it has failed to solve the problem, as games are still flaming out at the same rates. Any more ideas before I head out to get the main board recapped?