1995 Other Oldsmobile Models 1995 Delta 88, random engine st

I have replaced spark plugs, wires, spark coils, and had engine checked out by local dealer. Car will be running fine for weks at a time then will randomly stall when driving, no indication of issue prior to stalling. Runs good, smoth idle, good acceleration, no issues we can find. When driving car has stalled at low speed 20mph and at highway speed 65mph. Their is no indication anything is wrong prior to stall, just suddendle the engines off, all electrical still works, just no engine. When this occurs I just put the car in neutral, keep coasting with traffic, and restarts immediately, back into drive and keep going. Has happened 3 times in last 3 months and about 6 times in the last year. I have no clue, and aparently neither does the dealer. any suggesstions??