Most of us don't have a scope or the know how to use it. In my experiance there are a number of errors that show up with a drive fault, if it is, rebuilds ain't that bad especially given the price of the machine.
I would get the factory guys to help diagnose. They are helpful and free
Scopes to do most work on an nc machine are not expensive, you can find an okay one on craigslist for forty or fifty bucks, easy. 40 mhz is fine, for most stuff a 20 will do the job.
For simple troubleshooting they are not difficult to use, anyone who can write part programs should be able to learn it, and they are
hugely useful.
For instance, in this case, the symptom is that after a certain rpm, the spindle quits. (Or that's what we were told, I have noticed that people are not good about explaining the symptoms of their failure.) It seems very likely that the problem is speed-related. A common failure is the tachometer. Bad bearings, worn brushes, they can work fine at low speeds but crap at when you spin them faster.
A scope will show this. A multimeter will not. Tachs, ripple on various voltages, encoders, all kinds of trrouble-shooting is
way easier with an oscilloscope. It's not that difficult, and it's a huge help. Honest.
A lot of things the "factory guys" can't do. They aren't onsite. The more and better information you can give them, the better they can help you.
If you have an nc machine, you should have a scope ... it's that useful.