1990 Oldsmobile 88 Vehicle stalls while driving then wont s

My 1990 Oldsmobile 88 will stall while driving in cold or warm weather and at any speed. I have to pull to the side of road and try restarting sometimes it takes 5 minutes to restart sometimes it can take up to 3 hours. It sometimes will run fine the rest of the day and other times stall several more times per day.I have taken to local mechanic and replaced the catalityc convertor, fuel filter, fuel lines, and fuel pump tested at 50. We can not figure out the problem have researched online and seems as thow most say to replace either the crank sensor (not sure if they mean crankshaft position sensor) and/or ign(not sure what this is either)? ANY HELP WOULD BE GREATLY APPRECIATED AS THIS IS MY ONLY CAR AND MY MECHANIC CAN NOT FIGURE IT OUT.

THANKS