Trouble with Downcut

I’ve been using upcut endmills for a couple of years now with no issues. I seem to have gotten the hang of the S&F thing and generally get good results.

Yesterday I was cutting some plywood drawer dividers with an 1/8 flat upcut and was getting terrible chip-out, so I decided to try a downcut endmill I had bought several months back. I started with the same F&S and DOC I was using, and the dang thing went crazy. The only way I can describe it is like having a corkscrew in the collet. My material held fast to the waste board, but the entire gantry was shaking and the bit would not run straight. I slowed F&S and eventually DOC but to no avail. Finally switched back to the upcut mill and finished the pieces. I can see no obvious issues with the downcut bit. It is straight and dose not show any perceptible runout when inspected. Any suggestions?

can you post an picture of that downcut endmill (up close)?

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