This portion of your animation, frames 5300 - 5320, does not appear to have a lot of visual changes, i.e. large camera field of view changes due to changes in its position or rotation. I would think sampling more than 3 frames would begin to smooth out an GI flickering. For example, on frame 5300 you're currently blending the GI for frames 5288, 5300 and 5352 (assuming those gi frames exist). If you set the "Number of frames to blend" to 5, for example, then frames 5256 and 5364 would also be blended. The interpolation of those 5 frames should result in smoother GI.