88 Caravan 3.0 stumbles 10 seconds after coasting or stopping!

I have a 1988 Caravan 3.0 that has developed a problem where after you stop for a 10 - 20 seconds at a light or if you coast, the engine
looses all power and all but stops. The engine may be running at 200 rpm if that and sometimes stalls. If you floor the accelerator it will continue to stumble for 10-15 seconds then will snap your neck when it recovers. Will run all day @ a constant speed of greater than 20 MPH with good gas milage and acceleration. Just don't coast or idle the engine! I have noticed that the fuel pump seems to labor when the van acts up. The fuel pump and the filter have been replaced. I reset the computer and have driven the car over 200 miles and the problem persists. I received a code 12 and 15 from the computer. The van does not have cruse control and I have been told the 15 is related to a speed sensor that is for the cruse control. Any ideas?
Thank you, Randy
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