86 Deville shut down while driving, will not restart

Over the past month or two, my '86 DeVille (4.1 V-8, 175,000 miles) has been slowly losing power and mileage. It died once or twice, but was easily restarted. One day while waiting for a light, the car died and would not restart to its normal idle speed. The engine would actually run at a very low RPM if the accelerator was held about halfway down, but not fast enough to move the car. The engine is obviously getting fuel, because clouds of fuel vapor came out when the air cleaner lid was removed. When I grounded one of the plugs, I got a weak, white spark. I replaced the coil today, but it didn't make any difference. The plugs need changing, but that alone wouldn't shut the car off so suddenly. Could it be the computer or some other electrical issue?