The stock ones are thermal type, like the little metal can ones that are often found in cars.
These are quite voltage sensitive, and low battery volts will cause them to be erratic.
Try a solid-state flasher (3 terminal is best, use all 3 wires) for a nice, crisp flash. (Then you can also use LED bulbs!

)
3-terminal: connect the GREEN to the (-) or "G" terminal, the BLACK to the (+) terminal, and the grey to the "F" terminal (at least, on the 750s I have here, I don't have a 550 handy, but that will be close).