There are several known AMD problems
1. Later 486 chips implement RDTSC (read time stamp counter). This
doesnt work right on the earlier AMD 486's. Linux 2.0.27 or so
onwards contain a fix to avoid this problem
2. The earliest K6's have some very bad bugs. These mostly disappeared
after the first few runs but at least one random crash with lots
of memory was around until recently. AMD will swap faulty processors
without fuss.
Also note a lot of people throw AMD5x86 and Cyrix 686 chips into boards
that are not neccessarily up to the power, temperature or chipset specification
required. That is often easy to tell as the board worked fine with the
supplied CPU but not the upgrade.
Alan