Flashing lights on a heated grip controller is an indication of low voltage, a multimeter is your best friend here. I'd suspect the battery, a poor earth or a poor live feed. If the battery is good check the wiring between the controller and battery for voltage drop. If the lights return to solid when the RPM's are increased then you have low battery voltage and/or low charging system output. Check the battery, check the charging system.
Exception - Sometimes you'll encounter situations where low voltage can be expected. For example, sitting at a red traffic light engine idling at 1,200 RPM with head light on, indicators flashing and brake light on and the heated grip controller LED's start flashing, if an increase in RPM's brings the LED's back to solid that could be normal. What's happening there is that there is too much load and the battery and charging system cannot cope with the demand at low RPM's and the heated grip controller is letting you know about it.
edit: Just a thought, if you have ABS you may also have a stored ABS fault code (61 power supply low voltage). I explain
how I check and clear ABS fault codes here