It's happening because the seals in the transmission are worn out and when the get worn out they get hard and brittle. The seals keep the preasure up in the transmission so when the seals don't seal the transmission slips. It works after a while because the seals have had enought time to heat and swell up and when that happens they seal as they are suppose to. And you have no more problems until the next day,this problem will only get worse until the truck won't run at all. All you can do is get the transmission rebuilt,since just replacing the seals would cost almost as much as a new transmission. And you would still have an old transmission with new seals. I'm assuming it's an automatic transmission if it's not then it's a completely different problem you have. Good Luck
