2013 and memory slot 2 & 3 problem

Got a 2013 pcb rev 2.0 with BIOS ji438 worked for years with a single
PC100 cl2 128Mb dimm. Tried to upgrade to 512Mb recently with 2
identical Kingston low density (double sided) 256Mb PC133 cl3 modules
which the board is supposed to support. Only sees the dimm in the
first slot (farthest from cpu). Either dimm by itself works in the
first slot by itself. Single dimm in either 2nd or 3rd slot is not
recognized and gets memory POST beeps at boot. Even tried the
original 128Mb PC100 dimm, same thing. Ok in slot one, not recognized
in slot 2 or 3.
Sounds like 2 bad memory slots, huh? Replaced with an identical MB
from another system, and get the same results. WTF????
Reset BIOS settings to default, tinkered with BIOS memory settings, no
luck.
Any ideas?