Dimension 9200 Screen goes black during use.

As the subject says. During the use of my Dimension, the screen will go black and unresponsive. Originally started a few weeks ago, generally overnight or when none was around we we would only notice when we went to use it. Lately however, it will do this whilst in the middle of use. 

When it is unresponsive, the only solution we've found is to reset. Although it doesnt always come straight back up. It will boot to a certain point but then all hard disk lights and the like go out and nothing happens. Then is just a matter of resetting it time and time again until it finally comes back

Any ideas?