Feb 4 of #FFo3DP and #mostlyprintedcnc. Getting more confident in letting this thing go for 8+ hours for bigger parts. I wish I could eliminate the striations.
Since your printer uses helical couplers it’s almost certainly those. You could try and replace those with rigid couplers.
The best fix is integrated lead screws
As @Mike_Kelly_Mike_Make said, the fact that your Z thread looks like an 8mm is a problem too. You want rigid couplers so that the Z axis only moves via motor motion. And the threaded rod driving your Z should generally be a 5mm or 6mm – The linear rod should be thicker so it controls the Z motion only. With an 8mm screw; any defect in it can overpower the Z rods, causing them to misposition the X gantry over the part. Additionally, you want the threaded rod to float; if you have bearings at the top which are binding the threaded rod it will transfer even more into the X gantry.
I swapped out my M81.25 for single-start T82 and rebuilt my firmware for 1600 steps/mm instead of 2560 and that z-ribbing got a lot better. I also dealt with several sources of slop, including loose mounts for Z rods (wrapped kapton tape around the ends), loose linear bearings in the bed mount pillow blocks (magnets to reduce chatter until my new self-lubricating bushings arive), and now have almost no z-ribbing. T8*2 is hard to find in the US, at least; I found one Amazon supplier that it turned out shipped from China.
Check whether any of your parts have slop, and address the physical imprecision.