My Sonnet NuBus G3/240 certainly overheated. I bought it brand new and ended up sending it back for another one because it became such a problem with freezing.

Once I got the new one, I removed the expansion cover on the back and installed a little fan which I ran off of the CD-ROM power connector. It has worked fine since. I highly recommend at least removing the expansion cover if you don't have the AV card.