Just checking, when you're blending 5 files, are you blending file 0001, 0002, 0003, 0004, 0005 ... or 0001, 0006, 0011, 0016, 0021 ... or even 0001, 0011, 0021, 0031, 0041 ... etc?
Basically you should be rendering every 5th or even 10th frame for the GI Cache and blending those together so effectively the further apart the frames are that you are blending the smoother the GI light changes will be.