Over that 5 year period BYU had 7 regular season losses-4 of them in 1982 as Steve Young sucked as a qb that year. They finished ranked every year except ‘82. They were top 10 in 79-81, 83 and of course #1 in 1984.
IMO, the mid 80’s was when the cracks started to appear as other WAC teams started investing more in football and teams started figuring out how to defend the forward pass.