DIAGNOSIS The membrane switch panel behind the control overlay is failing. Moisture or wear causes poor contact.
FIX Unplug the blender. The control panel overlay is a single piece. Carefully peel it back from the edges. Underneath, you'll see the membrane switch pad. Blendtec sells this as a replacement part. Clean the contact points on the logic board with isopropyl alcohol before applying the new overlay.
IF THAT DIDN'T WORK The issue could be the logic board's touch sensor circuitry. This requires a full logic board replacement.
HEADS UP Do not use abrasive cleaners on the touchpad. Liquids can seep under the edges and corrode the contacts.