My '99 did the same thing. First time, it was raining really hard, got it towed to a garage. Mechanic said sometimes they will do this in heavy rain, and just put the belt back on ($70)! Second time, I drove over a medium to large sized puddle, and off it went. I was able to drive it home, and put a new belt on. The old one was a tiny bit stretched. Belts are only good for 3-4 years, so maybe that is the reason. Or, it could be: too loose or too tight, or your pulleys are not in-line.
good luck!
If it is a serpentine belt the idler pulley more than likely needs replacing. If it is a drive belt you are adjusting it too tightly.
The belt tensioner is probably bad.
Most likely one of your pulleys/accessories has a bearing going out and seizes intermittently causing it to fly.
A serpentine belt has an idler pulley on is to keep it tight at all times. If your serpentine belt is loose, then something is worn out. Its possible the idler pulley is stuck in its bracket so it cannot move or the spring on it is worn out.
belt tensioner is bad or one of your pulleys that the belt rides on is bad or out of alignment.
If your vehicle has a serpentine belt that is used to drive the accessories ( alternator , air conditioning compressor , power steering , etcetera ) the serpentine belt has a spring loaded tensioner that is supposed to keep the proper tension on the belt ( if it is working correctly )
There is a spring loaded tensioner that is supposed to keep the serpentine tight. If it is loose, it needs to be replaced.
There is no adjustment, they use a belt tensioner to keep it tight. If it is loose it need to be replaced.
look at your belt with the engine running (keep your fingers out) the pully on a little arm that bounces is your tensioner.
Your belt is either to big or just plain worn out.
replace the spring loaded belt tensioner as it's the only thing that will keep the serpentine belt tight.
The serpentine belt and the tensioner on a 300 SE motor is part of the fan system that controls all of the smaller motors on the front of the engine. The serpentine belt helps turn the alternator, power steering pump, water pump, and the fan. The tensioner is a pulley on this belt that helps to keep the belt tight enough to stay on, but not too tight to hinder operation.