How do you know the switch is fine?
A bad bulb would cause the side of the car too flash fast all the time. A bad flasher assembly I would think would just fail completely.
All the switch is, is a path to ground for the circuit. There are no components in it that would affect the current in the circuit in a random fashion. The fact that you can turn the directionals on and off successfully proves the switch functions. But before we completely accept it as good, do all of the lights have the same brightness? If not, then we might give the switch a second thought, but it's the ablsolute last thing I would condemn. I'd chace a ground before condemning the switch.
What's causing your condition is current variance within the circuit. The flasher is designed to open and close the circuit based on how fast the switch inside heats up. If all of the bulbs in the circuit are functioning correctly, the switch in the flasher heats up (and cools off) at a consistent rate. If one bulb goes out, there is less current being used by the circuit, so more is available to heat up the switch. This increases the flash rate in the circuit. If both bulbs on one side are bad, there is no variance in current availability, so the flasher stays hot and can't change from open to closed, so no flashing occurs at all.
Bulbs that have a broken filament may go open or closed with bumps. This is usually an older bulb that hasn't blown catastrophically, but worn out over time.
My first choice. Flasher. It's the thing that actually controls the rate of flash. Pep Boys sells one, but mine only lasted a year. Went back to Ford and the one I now have is 3 years old. My only complaint is it's too quiet.
Second choice. Replace all of the bulbs with new.
