Does your 550 have the throttle friction adjustment screw? Look for a large, knurled knob under the control pod. If you have one, back it out a few turns and see what happens. If you don't have one, look for any kinked spots. If everything looks OK on the outside, get a pressure cable luber. This clips onto the end of the cable and you attach a spray can to it, directing all the lube down the sheath. If the cables are still binding, it might be time for new ones.