I have a 2000 ECSB 4.8 bone stock.

The thing is just a dog, I don't remember if it was always like this but it can't even maintain 65mph going up a hill on the highway without shifting. It feels like I push the pedal in at least an inch and nothing happens until it downshifts.

The only maintenance done is oil change/filter every 3000 miles.

I've been reading about the GM top end cleaner, would that help at all. What other maintenance is recommended around this mileage?

THanks