99 Winstar deiseling

I bought a used Ford Windstar last year. It is a 1999 with a 3.8 l V6 and has 100,000 miles on it. It came with an engine with higher miles but after deiseling alot the crank bearings seized and rolled. The used car dealer put a newer engine in it but now again it is deiseling hard. What could cause this and how do I fix it? Shawn