DIAGNOSIS You've got a failing infinite switch (the knob that controls that burner's heat). The internal contacts get worn or burned and can't pass full power to the element. It's more common on the larger burners you use most often.
FIX Unplug the range. Pull the knob off the suspect switch. There will be a retaining nut holding the switch to the control panel. Unscrew it. From underneath (you may need to remove the oven vent panel under the controls), you'll see wires connected to the switch. Take a photo of the connections. Disconnect them. The switch pulls out from the front. Install the new switch (part number WB2X3522 is a common one), reconnect wires exactly as photographed, secure the nut, put the knob on, plug in, and test.
IF THAT DIDN'T WORK If a new switch doesn't fix it, the problem could be a damaged receptacle on the range top where the burner coil plugs in, or a bad burner coil itself. Swap the suspect burner coil with a known-good one from another size-compatible burner to test.
HEADS UP Make absolutely sure the range is unplugged. You're dealing with 240V terminals on the back of those switches. One wrong move with live power is dangerous.