I just noticed one of my K6-3+ is having problem with its BF2 setting. The CPU won't boot when BF2 is pulled high. On GA-5SMM the pin is set by a 2-state dip switch, so it's either high or low, and high won't work. On P5A-B the pin is set by a 3-pin jumper, so it can be high, low, or open. Again high won't boot.
According to AMD's document BF0,1,2 have internal pull-ups. The CPU can boot when the jumper is left open on P5A-B, and the frequency is indeed what is supposed to be as if BF2 is pulled high, but it will freeze during booting OS.
The CPU is normal and stable when BF2 is set to low, which leaves me multipliers of x4.0~5.5.
I have excluded the possibilities of overclock, overheat, improper voltage, etc. None of my other K6-3+ has this problem.