The temperature of the pasta water is really important for keeping your cheese from clumping. Following the steps in these instructions (releasing the pressure, turning off the pot, then adding the cheese) keeps your water at the perfect temperature to prevent clumping.  Another reason your sauce may be clumping is because of your cheese. Make sure to use a fresh block of Pecorino Romano and grate it yourself at home. Sometimes pre-shredded cheese will melt funky.  Also take care to grate your cheese on the smallest setting, as it should be as fine as sand. This smaller you grate the Pecorino Romano, the easier the cheese will melt.