2 memory modules & SPD

Hi, has anyone seen this before?

I am in the process of upgrading the memory in some Dimensions from the original 128MB to 384MB by adding one piece of 256MB PC-100 to each computer.

In one particular computer I got the following POST message on boot up:

SERIAL PRESENCE DETECT (SPD) device data missing or inconclusive.
Properly programmed SPD device data is required for reliable operation.
Do you wish to attempt to boot at 100MHz? Y/N

After answering Y the computer continues to boot and runs ok, but I get an abbreviated version of the message every time I reboot:

SERIAL PRESENCE DETECT (SPD) device data missing or inconclusive.
100MHz memory assumed.

If I remove any one of the DIMMs, either the new 256MB or the original 128MB module, the message goes away. I have tried several different 256MB DIMMs from other identical “happy� computers but it’s only this particular computer that complains.

I have even taken another “original� 128MB DIMM from an identical computer and tried it with just the two 128MB memory DIMMs and I still get the SPD warning. For some reason this computer doesn’t like 2 pieces of memory installed.

Any ideas on what could be happening here?

Thanks
Mark