You've hit on a common problem that often shows up whenever someone temporarily adds trailer lights to the flasher circuit; the flasher heats up too rapidly from the extra load and the flasher goes nuts. In those cases, a heavier duty flasher is required to match the load in order to slow the flasher.
67gta289 is dead on about the bulbs. If your bulbs have increased the load on your flasher because they have a lower resistance to current flow, they'll draw a larger amount of current, increased current flow means increased friction and the flasher reacts accordingly.
Isolating the circuit may net some results. If you have an Emergency Flasher, remove it from the circuit and then try your turn signals again. If no change, leave the Emergency Flasher disconnected until you've found the problem. If however, the turn signals now flash at a normal rate, then the problem is somewhere in the Emergency Flasher Circuit.
It is also possibly that you have a short somewhere in the circuit. Let's hope not as these can be tricky to find but I would do a visual of the wiring and bulb sockets first and then the wiring in the steering column and turn signal switch if necessary. You can try a heavier duty flasher and it may slow the flashing rate. If it does that may have been the only problem but if there is a short somewhere, the heavy duty flasher is only acting as a Band-Aid.
Have you tried just replacing the bulbs? It may be as simple as a bad bulb or two.
Also, I'll ask just in case...have you added any accessories to your car that you may have inadvertently wired into the flasher circuit?
Let us know what you find.
Ray
