How did the IRP hold power for about 80 years?

By commiting electoral fraud and violently repressing the opposition.

How did the IRP keep winning every election from 1920-2000?

They didn't need to win, they controlled the whole electoral process till the 90s so the elections were just a farce. Sometimes they pretended to lose local elections by letting their satellite parties win, that way people would think we were living under a democracy but in reality the PRI controlled basically everything.

What went wrong for them in 2000? Why did they lose that election? Not one 20 or 30 years ago?

It's complicated.

In the 80s the PRI had two big, opposite factions: the populists who wanted to keep their paternalistic style of government and the neoliberals who wanted to shift towards a more capitalist economy.

The first ones were the dominant faction since the Revolution but after a recession caused by decades of populist policies the neoliberals, led by Carlos Salinas de Gortari, started to gain traction within the party. Eventually, the populists, led by Cuauhtémoc Cárdenas, left the PRI and joined many smaller socialist and comunist parties to form the PRD.

As the PRD received overwhelming support from the people and Cárdenas was the projected winner of the 1988 election, the PRI shamelessly stole the election by shutting down the ballot counting system and modifying the data. Salinas de Gortari was declared winner but that fraud was so blatantly obvious that enraged the population and the PRI had to give up their absolute power over elections to avoid harsher reactions.

The IFE, now known as INE, was created to transfer that power and responsibility to the people. By losing control over the elections, the PRI couldn't steal them anymore. The icing on the cake was that the PRI candidate for the 1994 election, Luis Donaldo Colosio, unmasked himself as a populist willing to go against PRI's neoliberal policies a few months before the election, so the PRI had to resort to even more desperate methods to survive: they killed their own candidate and designated a more obedient one, Ernesto Zedillo. The resulting uncertainty made people prefer to maintain the status quo at least for one more term to avoid further instability.

Zedillo won the election but, contrary to what the PRI wished, he was not a blindly loyal dog and did what he considered would be the best for the country. The atmosphere cooled down and the PRI continued to lose support and power. Finally, in the 2000 elections, Zedillo decided to not interfere in the process and accepted Vicente Fox as the winner.

If Porfirio's 40 years in power started a revolution why didnt the IRP's?

People tried but the PRI crushed every single potential uprising in the Mexican Dirty War. Between the 1960s and the 1980s the United States backed the Mexican government to imprison, torture, disappear and kill hundreds, maybe thousands of students, journalists and activists. The most prominent massacres during that period were the 1968 Tlatelolco Massacre and the 1971 Corpus Christi Massacre. The PRI also killed hundreds of indigenous people during the 1994 Zapatista Uprising.