DIAGNOSIS: Food or liquid has seeped into the control panel around the pulse button, causing a sticky or intermittent electrical contact.
FIX: Unplug the blender. Wipe the entire control panel dry. Then, using a cotton swab dipped lightly in isopropyl alcohol (not dripping), clean around the edges of the pulse button. Press the button repeatedly while the area is damp with alcohol to work it into the switch. Let it dry completely for 10 minutes before using.
IF THAT DIDN'T WORK: The membrane switch under the button may be failing. If cleaning doesn't restore consistent function, the button assembly on the motor base is not user-serviceable and would require a base replacement.
HEADS UP: Avoid wiping the control panel with a soaking wet cloth. Liquids can drip down into the button seals. Always clean spills immediately and keep the area dry.