Serpentine belt problems

I have a 454 in my 1990 1500 S/bed. The serpentine belt mysteriously started coming off at high rpm and now comes off at anything over 2500rpm's. Very aggravating. I have replaced the belt tensioner pulley. Any advice would be appreciated.

Check the alignment of all of the pulleys. If one is even a little out of line the belt comes off. You may need to go 1/2 inch shorter on the belt length if you have changed any componient (alt,etc) and ended up with a smaller pulley. This is very unlikely in my experience. Good Luck.