TV Resolution questions?

Story goes. I've got a 120Hz 1080p 3D television, latest hdmi's, the glasses, the works. Out of curiousity, I hooked up the old style rca (red, yellow, white) and played a game, it looked like crap! I mean there was a rainbow effect when scrolling too fast & it looked horrible big time. But, i've got an old flat tube tv that I bought back in '02, it's 60hz. I hook it up to that one with rca's and the picture looks great, why?