Internal hard drive not detected. Sometimes.

Good morning all!

I have a strange problem with my macbook (mid 09, but it shouldn't matter)

- Four days ago my mac started freezing and I finally had to reboot.

- At the reboot I got the grey screen with a circle and dash.

- Reboot again: flashing folder.

- I plugged an external hd for backups with a partition with the installer of Mac OS Lion: the installer starts but it sees no internal hard drive

As three months ago another hard drive died, I have a THIRD drive with a clean installation of Lion on it (but in that case the "main" hd was actually dead)

- I plug the third hd and I get the flashing folder again

- I put the third hd in a box, connect it via USB and everything boots, still no internal hd

- to test, I put the first hd in another box and..... it BOOTS! AND.... this time it saw an internal hd (a forth one.. with no OS on it)

- then I put back again a bootable hd and it worked- I already tried to press well the SATA cable both in the hds and the motherboard

- this morning the OS started to freeze, so I reboot it, but I got the flashing folder

- now I am again with the external USB, after several others trials

So, to sum up:

- all the hds I have boot if connected via USB

- all the (2.5'') bootable hds I have also boot if installed internally, SOMETIMES. Other times I get the flashing folder. With no appearent scheme.

- all the hds look clean to distutil and smartmoontool (when I can run it).

- I already tried to reset PRAM

What could I do? What could it be? (Hint: I hope it's not the motherboard.....)

P.S.

I would not prefer to install 10.7.5 in this situation... also because I don't think it would change anything.