In a lot of these variable speed drives, the control's speed signal output has a voltage between 0 and 10 volts or perhaps -10 to +10 volts if the drive is reversible without using another forward/reverse input. The exact voltage on this input is proportional to the output frequency sent to the motor, thus the speed is controlled.
I have no idea if you can determine which control terminals on the Baldor drive are used for this speed input signal. If you have the Baldor manual, this should not be too difficult to find out. You can check this signal between that identified speed input and common, with a VO meter. If you use an older analog type meter with a needle, it will show you what is happening to this voltage in real time when the drive is acting up. If this particular input shows a varying voltage (when it should hold steady), then you know that the problem is in the control signal from the computer. This might turn out to be a component failure in the PC or a power supply going bad. That is my WAG.
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)