Assuming your battery and electrical connections are in good shape, if you're still using the original-style electromechanical flasher unit, consider upgrading to a modern electronic flasher.
The electromechanical flashers rely in a bimetallic strip inside the unit. As power flows through the strip, it heats up and bends, opening the connection. With power interrupted, it then cools off and returns to the original position, closing the connection. Because these flashers rely on the heating and cooling if a metal strip, and the rate of heating is proportional to the amount of current flowing through the flasher relay, the flash rate will vary depending on the supplied power and the load being driven. That's why it flashes faster when the alternator is generating power.
Modern electronic flasher units are much less affected by these factors so you get a pretty constant flash rate.
You can pick one up at almost any auto parts store, read the packaging carefully to make sure you're getting one that's rated to drive the number of bulbs you have (two per side). Make sure it's also rated for the amount of power drawn by your bulbs (LED or incandescent - the flasher unit I most recently bought could handle either).
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