If its a stock mechanical flasher, you probably have a corroded socket or a marginal bulb.
Inside the stock flasher is just a flat piece of metal that heats up , and it flexes back and forth to turn the bulb on and off. How fast it clicks back and forth depends on the current flow, since that affects how fast it heats up. If you have a bad bulb or corroded socket the amount of current through it will change, which changes the speed of the flasher. The most common problem is corrosion in the light socket or a bulb with the wrong wattage.
You can also buy a replacement electronic flasher, which has a small electronic timer built in. The flashing rate does not depend on current flow, so it will flash the same speed regardless of load (for example, when you plug in trailer lights).
Bruce