98 Ford Contour engine stops when

My 98 ford contour, 64k miles, 4cylinder automatic stalls at idle (engine just quietly dies) after high speed freeway driving, acts as if it timing is off when I try to restart it(kind of shakes and shudders) and/or it is out of fuel. After waiting 20 minutes or so, it starts fine, runs fine, sounds fine until the next stall out. I had the fuel filter replaced, Spark plugs replaced, chemical cleaning of injectors by mechanic and it still stalls after freeway driving of 30 to 40 minutes. No check engine light comes on when it happens or after it happens. Engine temp gauge is normal. What do I do now?