Not sure how you have it set up currently, but make sure you have the 256 stick in bank 0 and the 128 in bank 1, always put the larger first.
One of my stick has some faults in it, I actually have just a little short of 384MB....I forget which stick it is. I need to find the stick and pull it. That's what I get for paying peanuts for RAM.
Oh, and I run my CAS3 at CAS2....hehe.
b