My 91 Ford Aerostar has an intermediate issue. Sometimes it will crank over but will not start. I can drive it to work and come out following work and will not start but will crank. Had the coil replaced, new plug, new plug wires, New power relay, safety neutral switch and other things checked by mechanic. I had done in past unhook the battery for 10 minutes and then rehook he battery to reset the computer. I can drive the van for three days then come out to start the van and it will crank but will not start. Suppose to check the crank sensor but not sure they replaced this part. What can cause this Aerostar to crank over but not start.