DDR RAM : SPD device data missing or inconclusive

I have an Intel D915GAV MOBO with 1GB TwinMOS DDR RAM (two separate 512MB memory sticks, each with an effective speed of 400MHz). Recently I've added another TwinMOS 512MB (also with 400MHz effective speed) memory stick to the system, making a total of 1.5GB of RAM. Everything works fine except that when the system is booting it shows a message.

EDIT: Here's the message exactly as it shows:

SERIAL PRESENCE DETECT (SPD) device data missing or inconclusive.
Properly programmed SPD device data is required for reliable operation.
DDR 333MHz memory assumed at slowest timings.

I thought there was a problem with the 3rd stick since I never saw this message before when I had 1GB RAM so I removed it. But the message still didn't go away. So I tried each of the sticks one by one to identify the stick with the problem, but in vain. None of them had problems when used one by one, but whenever I put more than 1 stick into the system, that same message pops up. Now I'm really frustrated, and confused about what to do.

Is it a problem with the sticks or the MOBO?