1994 Oldsm Delta 88

1994 Olsmobile delta 88, 160,000 miles
My car will "die" when I am driving under 40 mph and my foot is off the gas- I lose all power and at that point only the brakes work. Sometimes it will happen 3x a day and then not for 2 weeks. I turn the car off and it restarts every time. This has been happening for almost a year and I have brought it to 3 different mechanics. Starter and alternator are relatively new - I have checked crankshaft sensor; transmission is rebuilt. Any ideas?