Have you looked at the plug in blinker module? Its only like $2 at any auto parts store (you know, they thing that makes that clicking noise when you have the turn signal on).
They flash based on the current flow. That is why when you have a burned out bulb, they flash really slow or not at all (not enough current to flash) and when you have a shorted or screwed up bulb, they flash really fast (too much current).
If you put the turn signal to the left and the front and rear turn signals light up at normal brightness but do not actually flash (and the same for the right side), I would replace the flasher module.
It is mounted in the fuse box and may require a 90 degree turn to get it out.
Edit: add a little more stuff I forgot about. Next time the turn signals stop working, try the hazards. They use most of the same wiring as the turn signals and will help you narrow the problem down based on if they work or not while the run signals do not.