my 87 cutlass supreme starts one min and won't the next

I checked the key switch, I checked the starter and the battery terminals. it still does the same thing. sometimes when I jump it off my truck it starts right up. so you might say it's the battery right? no the battery is fully charged and it stills does the same thing. it won't turn over or anything on a fully charged battery. it just started doing this like in January. a friend of mine told me that the distributor has a solenoid connected to it on the fire wall its supposed to help start the car but I cant locate it. any other suggestions are appreciated.