mumumu, the additional 4 memory chips borrowed from the other server was of the same kind as original Tiger503. So I do no think it is in-compatibility problem. Heat may be the issue. Our space in XO is getting hotter. I will ask XO to turn on an additional AC Unit to cool better. It might also be preasure. Tiger503 sits on bottom of rack, under all other servers. I will add a shelf to relive it of the weight from other servers....this can also hold in heat.