Ok, I have Nikon gear but for this issue I don't believe brand matters as it is pretty universal.
I have a Nikon D700 and SB-800 flash. I know the flash is faulty but not sure if it is just flashtube or bigger issue such as main computer board on flash.
These issues occur whether in manual or TTL modes and on or off hotshoe.
So sometimes it will fire fine at correct exposure, than wait a few seconds and fire off another shot and it will either not fire at all or fire very underexposed (2-3 stops under). Funny thing is sometimes the flash will indicate that is underexposed and other times it won't indicate that anything is wrong even though shot was 2-3 stops underexposed.
Take another shot and maybe it will be correct exposure again or it may be 2-3 stop underexposed.
In manual mode at say 1/8th power same deal. Fires ok, underexposes, etc.
I have a few other flashes and they work perfectly every time.
This is not a battery issue or shooting too quick in succession.
A flashtube repair is not too expensive ($80) but anything else and it starts climbing.
Do you think the it is just the flashtube?