check and clean the wheels the knife carriage uses to slide back and forth on the extrusion that the assembly mounts to,..also check the extrusion itself for gunk buildup(scraps of tiny vinyl and adhesive gunk from said scrap),.this may involve removing the top protective plate that covers the knife carriage bar.if it is on the wheels or bar it will raise and lower the head assembly slightly as it travels causing the blade to skip during a cut. any irregularity in the way the blade traverses this extrusion and or in the level of the of the cutting strip will cause discrepancies in the blade height and depth thus skipping in cuts can occur,...another cause is a loose blade holder,...