I have had the same issue of intermittent white flashing on the video output from my Denon AVR-3311C1 Receiver.
I am driving a 1080p projector with multiple HDMI input sources; Oppo Blu-Ray player, Mac laptops, Apple TV, Shaw PVR. All devices share the same issue of the video image flashing and and flickering intermittently when the signal is routed through the Denon amp. If i connect the HDMI cable to the projector directly to the source, there is a rock solid image with no flicker. My attempts to turn on the devices in a specific order Receiver>Projector>Source does seem to reduce the flicker frequency but not entirely eliminate it.
I sent the Denon unit back for service & it was returned with a whole new video section & a statement that they found a number of defects in the video board; but, this only improved it a bit at first and then we were back to the same old thing.
Today, I was thinking of heading back to my AV dealer for a new, different brand receiver when I thought I should make sure that I still have a clean video signal when connected directly from source to projector. I noticed on the Receiver that the HDMI output had 2 separate HDMI ports out. Since I wasn't sure how to switch to the HDMI 2 output I consulted the manual. In there it states that the HDMI 1 output has something called "HDMI Control" which allows the Receiver to issue commands via HDMI back to the source. The HDMI output does not support this feature. Hmmm, says I, 'Let's try the HDMI 2 output instead'.
Sure enough, using the HDMI 2 output totally eliminated the flicker. The video image is now rock solid and flicker free. I'm very pleased. After spending many hours trying to solve this, I now believe that the problem is fixed. Interestingly, the video signal passes through even when the Amp is off, which is not the case in HDMI 1 output mode. It may be that the HDMI control feature is responsible for corrupting the signal on HDMI 1 out and the more simple HDMI 2 channel is free of this.
So if you have 2 output channels & have the terrible Denon flickers...try HDMI 2.