Cuts getting every so slightly off causing them to get WAY off

So I’ve read a handful of threads on this (and I have posted a similar one before, but it only had one response and never got a solution), but I’ve tried many things and have not been having luck.

I’ll preface this by saying I’ve run into this issue in the past a couple times, but retensioning belts and V Wheels fixed it. This time around the issue isn’t getting better.

The machine: 1000mm XCarve with a DeWalt 611 (originally purchased in 2016, but have the upgraded X axis bar, the upgraded Z Axis, upgraded Controller, and upgraded motors/belts)

The project: Cutting various shapes (pumpkin, apple, Nebraska, etc) out of solid wood (elm, hackberry, locust, etc) that is around one inch thick (varies from .85 to 1.05 depending on the piece). I’ve done literally hundreds of these types of cuts.

The bit and settings: 1/4" Downcut Spiral Endmill bit, 200 ipm, 9 ipm plunge, .05 dpp (the same settings I’ve been using with this bit for years)

The problem: Two nights ago as I was doing one of these cuts it got down to the second to last pass along the river of a Nebraska board. The bit veered off path into the wood, leading to a 1" deep cut. (ruining that board and breaking a V wheel on the X Carriage in the process)

An assumption about the underlying symptom: I don’t believe the bit really veered off path that much on its own. It seems much more likely that the bit got off by a small amount, which lead to it bumping into an inch thick wall, which then led to it getting off by a lot.

Things I’ve tried:

  1. ensuring proper tightness of all V Wheels (including replacing the one that broke)
  2. retensioning all belts
  3. Resquaring the entire machine
    3B) As part of this resquaring, after I ensured the X axis was square to the table (not twisted forward/back), and ran a test cut with a 1" spoilboard bit. There was a slight ridge between paths along the X Axis, which indicated that the router was leaning forward slightly. I used a digital level to check that the Z Axis/router were parallel to the X Axis. It was off by .2 degrees, so I put a shim between the Z-Axis and the X-Axis Carriage to account for it. When I re-ran the spoilboard bit test it was beautifully smooth, which would indicate that the bit is perfectly vertical (or at least so close to perfect that any flaw is imperceivable)

Throughout this various steps I tried cuts again, though made sure I had my hand on the kill-switch every time just in case. Board 2, 3, and 4 got down to the final 2 or 3 passes (of 20ish) and had the same issue (though I was able to kill it before it destroyed the board). Board 5 didn’t have the issue (though it sounded like there was a little “chatter” toward the end of the bit cutting a little extra wood off path slightly). The next board had an issue as well. Somewhere along the line the issue got worse and the last 3 cuts only got about halfway through before the issue arose. (luckily I was able to hit the kill switch, then just used a bandsaw and flushtrim router bit to clean up/finish the cuts)

The only thing left that I can think to try is to recalibrate the trimpots for the motors (which are NEMA 23s).

Any other suggestions on what to try or ideas on what’s going wrong?