Problem with installing new Ram in MacBook late 2008

Hi,

I am having a problem with my RAM in my Macbook Core 2 Duo Late 2008 (Black). My computer started one day beeping 3 times in 3 second intervals which signals a RAM Bank issue. Then the computer began to show vertical lines. I took the Macbook apart and saw that many of the screw on the motherboard where loose if not removed, so i tightened the installed the screws again and that did away with the vertical lines. Then the next day it started doing the beeping routine again.

With some process of elimination i came to the realization that when i remove the RAM out of the left slot the computer boots normally.

With this in mind I figured that the ram was broken. So I purchased two 2gb ram modules with the proper speed of 667 and Im still having the same issue. With both installed the computer beeps 3 times in 3 second intervals, and when on the RAM is in the left slot the computer will book no problem.

I fear that I may have a motherboard problem but before i go and take the whole computer apart does anyone have another suggestion.

I have tried to resent the PRAM and System Management Controller but it has done nothing to help.

Any suggestions?