You said, "when you turn it off it lets one spark go from the coil lead...."

That tells me the coil is being charged.

In order for the coil to make a spark, first it has to be charged up (it's an inductor), then that circuit has to be opened, causing the magnetic field to collapse and driving the spark out of the coil.

Your system is for some reason not opening the circuit when it should.

I have had at least a couple of points type distributors that had the top bushing worn enough that the shaft could move sideways and the points wouldn't open. No Spark.

I have had a couple of electronic (non CD) systems where the pickup coil went open and so it didn't signal the box to open the circuit. No spark.

One of these things is happening to your car.

R.