If I may post your pic:
download/file.php?id=284&mode=view
The 6500's fan draws air from the case into the PSU and then out of the case. The gfx card is not really blocking the airflow over the L2, the RAM is. You might consider additional fan to assist and that would not really be a terrible Idea anyway. Even slow and quiet would be ok.
I am pretty sure that a Rage 128 is not putting much load on the system, though i suppose I could always be wrong.
In this thread we discover the 6500 PSU's ratings:viewtopic.php?f=12&t=13075
I have used three HDDs internally and a Radeon 7000 but no G3 in mine. There is another guy around who uses a Radeon 9200, G3, DVD, and who knows what else in his OS 9 running 6500(I think it is called Excalibur) with no issues. I am betting he uses plenty more power than yours or mine.
As far as perhaps a firmware or hardware bug? Yea, I would probably side with that before power or heat.
Sorry for jacking your thread Trash. :I