Heh, can't say I'm having any of those sort of issues with the 261 or the 248 drivers. With both the system was stable, though the 261 drivers ate the cpu cycles and killed bw.
Of course, this might be because I've just been upgrading Slackware from 8.1 through 9 to 9.1, with periods of CVS versions of Slackware stuffed in for the hell of it. Regardless, just looking at the output of make, it stripped gcc2 symbols from both versions of the driver. I've also patched the kernel with a few things, so that might have helped in my situation, but I've rarely had the magnatute of problems that you got, since I went with only what was actually in the system for the kernel, with a few networking options as modules.
It's nice to know that FreeBSD worked out for you, though it certainly makes no sense why Slackware 9.1 didn't like your setup. Either way, hopefully the other guy can get anything out of this mess of compiler issues