Полигон-2

STIW, судя по даташитам на микрухи кэша - они как раз то, что надо, если верить мануалу на материнку. Т.е. архитектура 32Кх8 и 8Кх8 для тэга - всё вроде ОК...
Наверное, всё-таки какая-то аппаратная проблема в материнке...
P.S. А что точно за ошибку пишет himem? Дословно?

Поглядел на свои 386 мамки, что с 8-ю слотами под симы. На всех стоят чипы кеша UM61256CK-20, то-бишь никак не меньше 256К кеша в них, вся память работает нормально, в обоих банках. Чипы тега IS61C64AH-20N.

STIW, по поводу той ошибки himem'а с сайта MS:

[q]

ERROR: HIMEM.SYS has detected unreliable XMS memory at address XXXX:XXXXh. XMS driver not installed.

CAUSE
This error may be caused by a fault in the computer's external cache controller or by bad or mismatched memory chips on the external cache (generally 64, 128, or 256K cache SRAM). If disabling the external cache does not correct this problem, there may be a problem with the motherboard at the current processor speed, A20 handler problems, or bad or mismatched main system RAM chips.

Use the following troubleshooting procedures to isolate the cause of the XMS test failure: Disable the external cache (also known as "secondary cache" or "CPU cache") in CMOS. For information about how to do so, refer to the documentation included with your computer or contact your hardware manufacturer.

NOTE: If disabling the external cache corrects the problem, replace the memory (SRAM) on the cache with the type and speed chips recommended by your hardware manufacturer. If this does not correct the problem, you may have a bad cache controller. Contact your hardware manufacturer for more information.
Disable Turbo in CMOS.
Disable Fast Gate A20 and Fast A20 Display in CMOS.
Try different machine switches with HIMEM.SYS (/M:2, 11, 12, and 13). For more information on this procedure, type "help HIMEM.SYS" (without the quotation marks) at the MS-DOS command prompt.
Replace main system RAM chips with the type and speed memory modules recommended by your hardware manufacturer.

[/q]

Это отсюда:
http://support.microsoft.com/kb/109845