Call Spectrum's tech support. Your issue is not with your cable boxes.
Pixelation stems from a signal issue (damaged coax, corroded or loose connectors, etc) that a technician is going to need to investigate. Call Spectrum's tech support, tell them which channels are pixelating, and request a technician to perform a drop-line test.
Replacing the box only creates a temporary fix. It does not fix the signals causing the pixelation & freezing.