Too much tounge weight could be a problem too. If the truck looks like it is sitting low in the back, it could be too much tounge weight. Try to center the load on the trailer so it only puts a little weight on the truck, but not too little. Negative tounge is bad ... very bad. (Dont allow the trailer to be pulling UP on the truck.)

I'd say check on the timing too ... but (did you say you had a '96 -- I cant see it now) you probably cant adjust it anyway. I know in my '93 timing effects its performance significantly, but i can adjust mine.

-IJF