The answer from Whirly Fizzle about Viewer texture memory buffer also helped me.
I had a similar problem where the background and surroundings were getting blurry, and then "something" went bonkers and updated the textures once a second.
This happened all over SL, my house, different sim's etc, even my self and skin/tattoo, was affected.
Incredibly frustrating and annoying to have that flickering in the background constantly. Strangely enough this did not happen with my alt in the same room/place, with the exact same settings and at the same time. I increased it to 1024 and all calmed down.
I tried it all, clearing cashes, changing graphic settings, re-installing Firestorm, nothing helped.
Thank you Whirly, i can now enjoy my SL again.