My car keeps shutting off while I'm driving. It's a 1990 Nissan Sentra sedan. While I'm driving it sounds perfectly fine, but after driving app. 40 miles it shuts off. My stereo and everything stays on, and since it's a stick everything still works except the gas pedal. The dash lights also comes on when it happens so I'm thinking it must be electrical? I can just pop the clutch and continue on my way. I'm worried that it's going to eventually leave me stranded. I changed the fuel filter first thing, and it didn't help. I read through my Haynes and couldn't figure out what it could be. I checked the ECM and no malfunctions came up. What could it be?