Phaser 7760 GX white diagonal strikes in yellow toner

Hi there,

My Phaser 7760 produces white feathered diagonal streaks in the yellow toner. So if I print a solid yellow square on a page you see white diagonal (45º) stripes a 2 cm apart from each other. Something is blocking the transfer of yellow toner on the paper. I shook the toner cartridge and cleaned the laser window. So far, I can tell it's not the drum as I tested swapping the magenta and yellow laser drums. Any ideas? TIA