Flicker on white screen when adjusting color/whitelevel

I'm posting this on the Software Support forum, though I'm expecting the answer *could* be power related.

I have 88 WS2801s connected to a Pi3 via SPI and a level shifter. I'm running a HDMI splitter and utv007 capture which works fine. My PSU is 10A and although it also powers the Pi and HDMI splitter I *think* it's powerful enough.

I'm supplying power at both ends of the strip. When I was previously supplying only 1 end, the LEDs gradually became slightly yellow-ish towards the end, but there's also always been a slight red tinge (on all LEDs) when display a full white picture. However the LEDs are stable - no flicker.

If I change the color adjustment settings - whitelevel, gamma, etc, even by the smallest amount (when displaying the white picture) - all of the LEDs start to flicker wildly. Which seems wierd - if I reduce the red value on "whitelevel" for example, surely the LEDs require *less* power and so should remain stable? But I get the wild flickering. So this got me to thinking that it's not a power issue.

But! I've found that if I set "Luminance gain" to 0.8 and therefore reduce the overall brightness, then I can adjust the color values without getting any flicker - which seems to point at a power issue!

So I can live with the slightly red hue to the colors or reduce the brightness. I'm happy enough to run with 0.8 Luminance so this isn't really a big deal, but I'd love to understand what's causing the issue. Any suggestions?

Thanks for the diagram!
So the one glaring difference with mine is that there's a few metres of cable between my levelshifter and the LEDs - I added the shifter explicitly so that I could have a longer cable length, but it guess it could be the issue. I'll remove the extra cabling and see if it makes a difference.
I guess I should look at adding extra 5v cables too, it can't do any harm... though I'm measuring 5v (+/- a few percent) throughout the stip. Ha, that said I don't think I measured while the LEDs were at full white, so definitely worth re-measuring.