I just purchased a couple of Flex TT5's and a MiniTT1 (with an AC3 zone controller) to use with my Nikon D700 and 2 flash heads.  I have an SB800 and and SB910.  For the most part, things work as expected.  However, every 7th or 8th shot, for some reason, my exposure blows out, as if the flashes are just firing on full power, way overexposed. 
This seems to happen whether shooting TTL or manual flash. 
I'm trying to control for variables.  I'll shoot a posed subject, on a tripod, not moving the focus point so that all exposure variables remain constant: metering, aperture, shutter speed, ISO, all the same.  This problem still happens.
I believe the SB800 is the one that fires full power now and then. 
Any ideas?
My PW's are brand new and have current firmware.
Also, I unhooked the PW's and tried shooting wirelessly with the Nikon CLS infrared system and the problem didn't happen, so I'm thinking it has something to do with the signal coming from the Flex to the SB800.
I'm afraid something may be wrong with that flash head, although it's never given me any trouble and it's at least five years old.
Thanks for any help...