First post for me. Would appreciate any help from you all. Our 1999 Suburban with the 5.7 began popping the check engine light on and off last spring. Over the summer, on a trip, it began running rough under load (up a hill or around 50-55mph). It has gotten worse and only occurs after the truck is heated up on the road. Doesn't do it cold or when idling. Had code read from O'Reilly's and it is P0430. I am hoping it is not a cat. Could it simply be a O2 sensor? Would that make it run rough? Any thoughts on where I should start would be great. Thanks!