Yea, I was puzzled by Bruno's problem statement too. But I am confident he means that they blink too fast (a common problem) - not that they only work above 85 mph (which would be pretty amazing).
Most likely, there is nothing wrong with the flasher. Replacing it with a new, mechanical flasher is unlikely to fix anything (*). Fast or slow flashing is usually the result of a burned out bulb, corroded socket, bad ground or some other condition that has changed the resistance of the flasher circuit. Do both the L and R sides flash equally slowly? Have you walked around the car while the turn signals are on, and observed that each bulb is working and glowing equally brightly?
*) As RobGTV wrote, an electronic flasher will solve a lot of problems. These flash at the same rate, irrespective of current draw. They work the same way when bulbs are burned out, incorrect bulbs are installed, LED's are substituted, etc. So replacing the old-style mechanical flasher with an electronic unit may well solve Bruno's problem.