I'm sorry, I'm having trouble understanding exactly what you've described.
My question would be, what kind of cnc control does this machine have? Does it involve PLC ladder logic? If so, it sounds as though something has been corrupted in the PLC ladder, or else, it never was right to begin with.
Part of the problem involving a rapid slowdown, then coasting for 15 seconds would be considered "normal", except that its not ideal. The spindle drive parameters are incorrectly set, the deceleration time is too short, so the spindle drive does not have enough time to absorb the energy required to stop the spindle. The drive then "cuts out" before the spindle has stopped, so now it has to coast. The PLC ladder logic watches for a very low or zero speed signal from the spindle drive, before it then allows further execution of the nc program.
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)