Downcut Bit Leaving Very Uneven Cut

Any advice on why a 1/8" downcut bit cut as shown with a very uneven bottom of cut. It varied on the 24" piece of hard maple from 0" to 0.05". The 0.25" cut was down at with a feed rate of 75 IPM, depth of cut at 0.05", 0.5 stepover. First time it has happened. I used an 1/8" upcut from an additional 0.05’ to clean it up with the same settings and it worked perfectly. Thoughts?

Is the machine in good mechanical condition and the spindle secure and the tool/bearings in good condition?

What is the total depth of cut?

What is the depth per pass?

One easy way to attempt to address this is with math, for example, if cutting to a depth of 0.5", while depth per pass is 0.125" and making four full depth finishing passes, one can reduce the depth per pass to 0.124", which will result in 5 passes, the last a finishing pass which will only remove 0.004" of thickness.

What Machine and Z Axis do you have?

This looks like you have vertical play in the Z Axis.

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