I have a FFCP (same year/make) and I have LOTS of post on this forum regarding the same thing or very similar. I came from the old "woody" FF and another self built printer and had no issues but when I bought my FFCP I had nothing but banding and weird lines and other issues!
I posted here and also on the FF Google forum and had tons of good suggestions but the problem ended up being loose and poorly built X/Y molded rod holders for the gantry. There was a small amount of slop in the molded black pieces that the chrome rods go into that hold the main head assy and that slop was causing the issue!
But before I found this out, I changed out TONS of stuff, bought 10ft of GT2 belt from Amazon, bought new NEMA 17 steppers, bought new drivers for the main board, swapped out the main board from a buddy's FFCP (after I convinced him).. bought a new Z stepper with the rod (because the Google group said it was prob slightly bent- another $99)... tore the whole machine apart and rebuilt it top to bottom and used locktite... still had the issue.
In the end it was the rails, and there was just enough slop to cause issues. I ended up using 30min epoxy and put everything nice and tight! It's never coming apart or loose and I get amazing prints close to my Mark Forge because everything is rock solid!
One thing to note, if you're using USB, do a test print using the card first! some of the boards had a bad interface and you could be getting lag.
I can take some pics if you want but your issue sounds like a slop or variance issue (aka mech)
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