I don't mash. I queue my skills, and the only way not to cast SS on proc is to move as you see proc appear on the screen which still interrupts the cast because there's a slight lag before it appears on the hotbar. Time wasted on that dancing around the problem wrecks the timing of off-GCD skills. If it's still not the problem for you, try watching out for procs while keeping an eye out for healers' MP bars (and soon for everyone's TP bars as well). And simultaneously watching out for EA to be off CD. For BL to suddenly proc. For healers suddenly moving out of 20 y radius. For AoE's.
SMN, the only other job working on priority rotation instead of combo rotation, don't have to do nearly as much. MCH don't have this problem with procs wrecking their timings since they can force procs.
