Unstable Lion

Since installing Lion, my Mac Pro has been forcing me to shut down at least once a week. Anyone have any clues?
All apps.

Shuts down the whole machine even if I am in the middle of anything no matter what it is. Screen goes dark gray and the black translucent with white lettering comes with the warning that I must shut the machine down by holding the button down.

Need more info ,certain apps or all apps ?

Since installing Lion, my Mac Pro has been forcing me to shut down at least once week.
All apps.

Shuts down the whole machine even if I am in the middle of anything no matter what it is. Screen goes dark gray and the black translucent with white lettering comes with the warning that I must shut the machine down by holding the button down.

That's a kernel panic. The next time it happens, copy the panic log to the clipboard and then post it in its entirety here. We'll take a look at it and see if we can find out what's going on.

And most KPs are hardware related and often related to faulty memory. That is why the log is vital.