My laptop monitor shuts off randomly, but computer still runs, why is this?

My Dell Vostro 1500 notebook on Windows Vista Business 32 Bit has been acting funny lately. The computer is a lil over 3 years old now and this weird thing where the laptop monitor randomly turns off, but computer is still running. Is there a driver thats corrupt that i need to reinstall, or is it a hardware issue? After the monitor goes out i close the lid to put it to sleep, then its perfectly fine after i open it up (login screen appears, etc). If i run the computer on its battery only, this problem never happens. Whenever its plugged in, it messes up. Please help!! *Here are my computer specs if you need them Dell Vostro 1500 on Vista Business 32 bit 2 gigs ram nvidia gforce 8600gtm 120 gb hard drive space lcd monitor with (w/o true life) no error messages with this problem at all. I just hope this isnt a hardware issue because i am soon going to buy windows 7. =/