AX4C Max unstable at FSB over 176MHz

Hello,

I purchased a (supposedly repaired) AX4C Max a while ago. It runs fine
with a Pentium IV 3.06GHz Northwood (533FSB).
However, I tried it with a 2.8GHz (800FSB) CPU, but it is completely
unstable at this speed. CPU is fine. When I drop the FSB to 704MHz
(4*176) the system is stable again, but not at any speed over that.

Is this a well known problem? AOpen will not RMA this board and
neither will my supplier, since I bought it with limited warranty.
AOpen Support was unable to reproduce this problem on their system,
but their mainboard was from a newer batch.

I tried changing RAM (5 brands), changed power supply (Zalman 400W,
Enermax 265W, AOpen 250W), changing VGA card, etc. The only conclusion
is, that the mainboard is at fault. This was also the conclusion of
AOpen Support. I am currently using the mainboard with the 3.06GHz CPU
again.

Do you believe a mainboard repair specialist could fix this problem? I
myself have no idea what to do about it or what the exact cause of the
problem is.

Any recommendations are welcome!