Gray screen and folder with question mark

I have a 13" macbook mid 2009 unibody white 2.13 ghz model, me too came across this issue of folder with question mark on startup, so after googling out and referring to different forums I came to know it might be due too,

1) hard disk failure (corrupt hard disk).

I tried to remove my hard disk and inserted it into other macbook, a older version mid 2007 (black color) and I found the hard disk is perfect, as I was able to login to older macbook and access files without any trouble.

furter on exploring I learnt

2) Trouble may be due to faulty cable connecting hardisk and logic board.

I tried to replace the cable with the one from the old working mac book as well as other macbook cable of same make from same version macbook but it couldnt solve the issue.

further going I read that the problem may crop up due to RAM.

3) I removed both the RAM(1gb each) from the troubled macbook and checked it by inserting in old working macbook it was working fine, so I took one of the RAM from older macbook and inserted into the troubled macbook with startup issues to my amazement it worked , I was able to see Apple logo and was able to successfully login, I accessed my folders and internet then just to confirm whether the issue is sorted out forever I tried to restart the macbook ---- the old issue cropped up i.e the folder with question mark was there

I tried to remove this RAM and replaced with other single RAM from old macbook again it started and was able to log in but when I try to restart the machine It comes up with the same issue I changed 3 different RAM's and was able to log into macbook om the first instance but when I use the same RAM to restart or start macbook I have that ISSUE cropping up I am very frustrated, please some one help me to over come the trouble I'm facing.

I have already tried resetting PRAM and other commands using option, shift etc.

The macbook produces CHIME on power on then gives the folder and question mark.

Regards,

Krishna