A 3 or 4 flute center cutting endmill does not exhibit good chip clearing ability when plunging. They are better at ramping down into the cut, on a boundary ramp or a helical ramp.
Even two flute endmills are "iffy" when vertical plunging. If you look at a two flute endmill, you will see that the most common grind for the center cutting edge, creates a perpendicular face that does not mimic the helix of the spiral flutes one bit. This can and does plug up, and causes much shuddering and shaking.
I like to carefully grind (by hand) a bit of a back rake into that perpendicular face. Be careful to not actually grind the cutting lip back, just grind up to it and stop. Preferred wheel would be borazon cup for HSS tools, or natural diamond for carbide, because these wheels will hold their shape and you need quite a fine corner on the wheel to grind right to the center of the endmill.
Avoid the dust when grinding carbide and HSS.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)