I've had this truck for about 4 months now and I've noticed a problem that's gotten progressively worse.

Problems- When going uphill my transmission will keep shifting between 2nd and 3rd gear and will sometimes shift out of OD for a few seconds then hard shift back.

I've checked the fluid because to me this seems like there's not enough and that's whats causing the issue, but whenever I check the level the stick is always completely covered like there's way too much fluid.

I talked some family who's had a transmission shop for 18+ years he said it was because my tires are a size bigger than standard for my truck it causes the shifting. But if this was the case then it would of done this since I've had this truck. I talked to the guy I got the truck from and the only problem he ever had out of the transmission was it would shift hard sometimes and not any of this.

Can I get some help here?