Ive had this several times, but only in the last few weeks and only in the yak9u
i will be in a fight turning and such, "blackout" creeping in or out during the manuevers.. the AFTER all of this I make a small relativly low G turn at medium speed and WHAM.. everything instantly black.. and it stays this way for a fairly long time, the better part of 10 seconds.. the darkness wasnt "closing in" or anything, i had full FOV and made a minor adjustment 10 degrees or less..
then i start to come back and I turn my head just a little (track IR) and WHAM back to total dark.. no movment of the stick was made.
if I haven augered by now im in for another 10 sec or so of darkness.
ive never had this problem until recently