I had the same problem on my ZRT last year it would top out at 110mph or so. And this year it seemed to run fine and then I went out in one of mybosses hay fields and the most I could get was 80mph for a second or two than it would start to fall
But when it was in the 40-45 degrees it would go like a bat out of he!!
Some one told me that it could be the stator going out but the sled on had 2500miles on it and it was a 97! I never did find out ,had a little engine fire and had to total the sled out
I just bought a 02 pro-x fan , you could get 90-95mph out if it, what mods did you do to it? Also if its a 02 or 01 shouldn't there be a warranty on it? I would let the dealer worry about it!