I don't believe Michael was ever slipping. His cold heart is what kept his mind clear of all his decision thinking. If Michael was slipping, he would've kept Fredo alive, he would've trusted to more and the rest of his associates. Vito trained him very vell before his passing.

I'll admitt, after Fredo's betrayal it definately broke his heart, it was his only blood brother left. Thats why he loved Fredo so much and always protecting him but after that betrayal incident, how could he trust Tom and and the others. Michael was all strictly business, thats what I think kept him from slipping.