Memory Install Slows System

I'm having an issue that I've never seen when adding memory before.

Intel Celeron 3.2 GHz
1 GB RAM came installed. 1 stick in the 240-pin slot (DDR2 SDRAM 533 PC2 4200)
Motherboard is an ECS P4M800PRO-M478 (v1)

Motherboard shows max memory of 2 GB

I added another 1 GB stick. A different brand (Corsair ValueSelect), but identical specs.

The memory is being recognized at startup but the system is running very poorly. CPU usage spikes at 100% and everything runs slow.

There is no ventilation problem. Inside the case is clean. All fans running. There is no svchost mem leak, that is not the process that is maxing the CPU. Whatever program is opened immediately runs the CPU Usage to 100%.

When I remove the new memory stick, the system runs fine.

What am I missing? I'm FAR from a computer specialist, but I've never had any problem installing Memory before...

I'd really appreciate any help that I can get. I'm guessing it's probably something simple that I'm not getting.

THANKS for everything!!

 

Yup. Exactly what I was going to suggest. Both sticks might be OK, but the individual timings may conflict....

 

Good ideas....I should have suspected the mobo right off the bat. But apparently it isn't the culprit. In fact there may be no culprit.

Anyway, the new stick alone performed perfectly in the slot that housed the original RAM. When I added the older stick into the second slot (switching the positions of my earlier install), everything fired up fine, system is running smoothly and the CPU usage is normal. Go figure.

Thanks so much for your timely replies. Your help is greatly appreciated.