Getting a body and lens combo to work right through older firmware?

Hi all,
I seem to still have trouble getting my Tamron 24-70VC to behave properly on my 5DIII. On my 6D it works perfectly, but on my 5DIII it has some pretty awful focus inconsistencies even in perfect lighting situations. The lens was sent to Tamron a few months back to be checked (they replaced the VC unit for some reason) and like I said, the lens performed excellently on my 6D (and my previously owned 7D). I've run out of things to try except to maybe try to revert the firmware on the 5DIII to a slightly older version to see if it makes any difference. Am I wasting my time? Anyone know where I can find older 5DIII firmware files? Any other suggestions?
Thanks

Any thoughts or ideas?

If the focusing inconsistencies are always front focusing or rear focusing from where you place the AF point, I would suggest doing AFMA on the lens with the 5D3 and see if setting an adjustment value at both ends of the zoom reduces or eliminates your issues. AFMA is the in-camera feature to micro adjust focus for up to 20 lenses, with a setting at the wide end and telephoto end of a zoom if needed.

I've attempted the micro focus adjustments but to no avail. It isn't a consistent front or back focusing. However, it does seem to be more of an issue at the telephoto end of things. I'm rather confused. Part of me is stubborn and wants to get to the bottom of it; whereas the other part of me says screw it, ditch it, suck it up, and fork over the $$ for the Canon 24-70II.