Mercury topaz Problem

The car was running then all of a sudden the engine was not running and the car felt like it was in neutral, but it was in drive. Coasted to a stop after switching to neutral. The car would not restart. After it sat for a good 15 minutes it fired up and i proceeded down the road only to make it 500 yards to have the rpms drop out and the coast deal again. I changed the fuel filter thinking that was the problem. Started the car up and it ran for about 8 min then the engine cut out again. At this point the engine will stay running if i keep the starter going to help it, but as soon as the key gets turned back to the run position the rpms drop to 0.
What could the problem be???
Thanks guys.

My son's tempo had a similar problem. Traced it back to two bad spark plug wires. So I put on new rotor, cap, wires and plugs, then it ran fine. Try a timing light on each plug wire as close to the cylinder head as you can get to see how they are firing. Or you can use a plug wire puller, removing and replacing each one while running to see what effect it has. If taking off any doesn't cause the engine to start straining, then that cylinder is probably the one with a problem. Also, a lot of auto part stores will now read the codes off your car for free, so check around for one. Make sure you use Autolite plugs if you do change them. The Ford's just don't like to run well for very long with the Bosch plugs.