5 processors in the range of 400 MHz? I am assuming you changed few heatsinks as well.had a 1700+ pally, then a 1900+ pally, then a 2100+ T-bred, then a 2200+ T-bred, then a 2500+ barton.
My mother used to say -- when you buy cheap you always buy twice, once because it is cheap and the second time because it was no good. Well, 5 times beats that.
Try new Bartons, heard they overclock 100 MHz higher than the older ones, they might even have a better heatsink.