Probelm when write/read DDR2 memory

I have problem when I try to read/write the DDR2 memory. I am using alters's DDR2 controller, and use NIOS to controll it.

If I only write 1 single data to the memory, and read it back immediately after writing, it works, but if I tried to read the same location some more times, I may read the wrong result data soon.

It seems the higher 16 bit of the result data is either the higher 16 bit or lower 16 bits of the correct data, the lower 16 bits of the result data is from the data in other location.

And sometimes, there are a couple of bits error within the 32 bits result data.

Could anybody can give me some hints about this?