what causes loss of power with high rpms and downshifting during passing attempt on 1999 GMC Suburban 1500

when stepping on the gas to pass at 65 mph, the tranny downshifts, the rpms run to 4500-5000, yet there is complete loss of power and acceleration. Also happens when climbing steep inclines. Cat. convertor just replaced.

Asked by for the 1999 GMC Suburban 1500
sounds like your transmission is slipping