Continuing 'permissions error'

For over two months now I have had an ongoing error with my macbook pro which was originally running 10.7.2

The first time I noticed the error was when I went to empty the trash and the following message appeared:

"The operation can’t be completed because an unexpected error occurred (error code -50)."

Once I reset the system I was able to empty the trash normally until the error occurred again and I then noticed that I could no longer create new folders, RSS feeds didn't refresh and emails would not download. Again once I reset it would work normally until the error occurred again.

Basically the best way I can describe it is that i'm logged out of the system without being sent to the login screen.

I have taken my mac to the genius bar where my HD was replaced and a clean install of 10.7.3 was carried out, within 24hrs the error reoccured.

I have verified both User and Home permissions to no avail.

I have created a new user and brought nothing across and the error persisted.

Over the last 2 months I have re-installed Lion numerous times and have slowly brought backed-up documents across to see if it was a particular file causing the error, trying this is numerous orders was in-colnclusive.

At a suggestion of an Apple Premium Service Provider I am now running a clean version of 10.6.8 and have not brought anything across from a back up and the error has still occurred.

any suggestions welcome?!