2000 Buick Regal 2000 Buick Regal GS intermittently won't s

Engine Mechanical problem
2000 Buick Regal 6 cyl Front Wheel Drive Automatic

Since the weather got cold my Regal intermittently won't start - it cranks fast and clean but won't catch. When 20 degrees or below it won't catch at all; but once, when it was 36 degrees F, the engine would make a gurgling sound when I cranked it as if it was trying to fire but wouldn't start. When it's running the engine sounds and runs great. I towed it to the dealership once after a no-start and when they got it it started instantly. They couldn't find a problem. A second time I drove it in after the no-start happened again and it started right away and they couldn't find a problem. Can you help? I keep getting stranded places with a car that won't start.