My Sabaru's engine dies suddenly...while car is being driven?

Hello, I have a '91 Sabaru Legacy. It's a great little car and it seems to run great, except the engine dies suddenly while I'm driving. I can be going 50 MPH or just crossing the parking lot. I changed the fuel filter, but that hasn't fixed it. It starts right up and runs smooth as silk, but suddenly while driving, it's as if I had turned the ignition switch off. Then, the dashboard lights up, just like it does when I turn the key on in the morning. This is a completely new problem -- as it has run trouble-free for 2 years, ever since I bought it used.

BTW, it usually starts right up again.

Please help! Thanks!