some motherboards have different speeds for different configurations i had a asus with a 3400 and with 2 slots full it ran max 400mhz when you put in three it would go to 333, look in the manual for the speeds.
Doesnt it usually go off the ram speed of the first slot??? so if u put a stick of 400mhz ram in ure last slot and ure first slot is only 333 it will reset to 333..^