60 - 120 times per minute, so it must be going fast to fail!
All bulbs are obviously working? If so, I'd go for a new flasher unit - they don't last forever and usually end up fast/slow or not illuminating the dash light (which is also a fail).
i had this problem just the other week, but only on the right side. found that the right rear indicator wasn't flashing aswell, so took out the cluster and checked bulbs and tried again, worked fine . have had problems in the past with my left rear light cluster wiring and it turned out that the indicator would tick/flash really fast if the rear indicator wasn't getting signal. tucked everything back in carefully and working ok now.
i assume all your indicators were flashing? doubt you're having the same problem but hope it helps