I have a Canon 420EX Speedlite that sometimes won't start up. When it's not working, it behaves as if it didn't have any batteries in it... just totally dead. The odd thing is, it doesn't seem to be the batteries. The ones in it are fresh and meter well; I've removed and replaced them several times, even tried a completely new set of alkaline batteries (I usually use NiMh or Li rechargeable). It doesn't matter what I set the switches to, it doesn't matter if it's on-camera (a 5d2) or not — it's just dead. But leave it sit for a few hours, and then it fires right up and is good to go.
This weekend is a perfect example. I tried to use it Saturday night and it was totally dead. Swap the rechargeable batteries for a fresh set of alkalizes, thinking I had left them in too long between uses and they had evaporated, and it was still dead. Swapped back to the rechargeables after metering them and seeing they were pretty fresh, and it was still dead. So I left it sitting on the dresser and shot with ambient light. Next morning, trying to shoot the kids, snapped it on my camera without thinking and it fired right up and worked fine... I got off 60+ shots and only missed flash twice because I was shooting too fast.
Anyone got any idea what the problem might be or how I can correct it when it happens, other than leaving the flash sit for hours? Or a way to prevent it would be better.