My truck is a 2000 F150 4.2 auto, its having this problem shifting. Sometimes when the truck shifts to 3rd the truck just revs for a couple seconds( like its in neutral) and then shifts. But if you let off the gas a lit bit it shifts fine to 3rd. Any ideas?

 

Any help at all? Im a 15 year old that will be driving this truck around when I get my permit next week and my uncle is a mechanic so he can fix it.