I got an old Rockwell TS last year, and it has never ripped thin boards perfectly straight--they always came out slightly banana-shaped.
Last night I was making the fence for a drill press table, and it finally pissed me off enough that I decided to figure out what it was and fix it.
Turns out that the fence thickness varied, and was substantially thicker at the front and back than it was in the middle. This ended up giving the effect of a curved fence, which thin boards would naturally follow.
I ground down the steel fence front and back, then cut a scrap of melamine to size and screwed it onto the fence, shimming the middle out with enough layers of paper to bring it flat.
Works great, and now I'm kicking myself for not doing it sooner.
Chris