starts and looses power alternator and battery is good

What is your question?

My 2000 ford Taurus losses its power after I start it and it will die. I have changed the alternator twice and the battery. When it dies or if I start to reeve up the engine it blows the mega fuse, on the side of the fuse box under the hood. I have been told it's a short in the system or something is not properly grounded. I was wandering if anyone has had a similar problem and could tell me where to look any help would be pretreated. Like I said it starts and looses power alternator and battery is good where should I look for the problem thanks.