1 and 2 memory 3 blank 4 memory
This does not sound right - right in the sense that it works but not otherwise.
It has a dual-channel memory controller which means that memory must go in pairs (of equal size and performance) - if not all memory changes to single channel and much lower performance. So much so that you get a real boost to remove the odd memory unit
This may also be the cause of the speed problem
(This is not the case for Athlon on s754 which all are single channel)