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EOS-1D Mark III Err99


I have an EOS 1DMk3 that I purchased second hand many years ago.  It will work fine for a while and then throw an Err99.  I turn it on and off, I swap batteries, and still, it's Err99.  So, I park it for like 6 months, put a newly charged battery in it and then it suddenly works.  It will work perfectly for a while.   Then it throws Err99 again, but if I keep trying to shoot eventually it starts working again for as long as I use it.  When I turn it on again the process of Err99, continual shutter attempts makes it work again.  I know the camera is no longer serviced, but I'd like to know if there is anything I'm missing that could make the camera a more reliable shooter?  Regards, Tim Murphy