Memory amount changes

[christophar1]

I have a T43 2687 that was given to me. I found it showed 512mb of memory even though there was a 512mb stick under the keyboard and a 1gb stick in the bottom slot. I removed the 1gb and placed a 512mb. System recognized it and worked for about 1.5 hours. I then had to reboot and it switched to only showing 512mb. Powered computer off, went into config and it once again shows 1024mb.
Any idea why it would change or incorrectly read installed memory?

[dr_st]

Possibly the bottom slot has a bad contact, which makes it sometimes detectable and at other times not.