What could cause my 1999 Sable Wagon to drop rpm's and shut down.

I changed out the coil pack, new plugs/wires, etc when the car was misfiring. Driving the car with work on a hot day, it overheated.  Got it to the shop, showed the coil pack needed changing again, under warranty so did it.  Driving again with work(use car all day with work house to house job), all the sudden rpm's drop, car jerks forward 3 times and car shuts down.  This happened three times different times on the road after the shop checks it with no answer to what is overheating. Get an error reading of p0320. Four different people discussed this and with the readout went with replacing the Crankshaft Sensor.  Did not want to start again so the checked wires to this (which they were suppose to do when they changed sensor). It ran just over 3 miles starting and shut off engine at 2 different stops.  Got out of parking lot after sitting for 3 hours, did not get 300 feet on the street and it jerked forward 3 times and rpm's dropped and shutdown.  Will not start no matter what.  It cranks, but will not turn over.  Please someone any suggestions. My regular mechanic has it back this time and he just did a preliminary on it, but he thinks it seems like it is not getting gas to the engine. But with what this thing has been doing over the past 4 months and the error readout possibilities could this be more than one things still. Please help, have to have car for job.  No car no money coming in.