how to test a speedlight for issues?
When the 540EZ flashes were top of the line, Canon reps told me not to shoot the flashes in multiple bursts. I used to fire them like 10 repeated shots at a time and I did get wide variances of exposures. Canon tech reps told me this is normal because the circuitry has to have time to recover.
I started firing my flashes at bursts of 3-4 maximum from then on. I told Canon they should design the hot shoe and flash boards to handle extensive flash firings but they said the capacitor would melt the circuit boards.
When I need to fire my flashes repeatedly I have a few sets of batteries ready and just switch them out but you also have to let the flash rest between firings. I know it is a hassle but those flashes weren't designed to fire off shots like a Speedotron or Metz.