"速度忽高忽低" can be translated into English as "the speed fluctuates wildly" or "the speed varies erratically".
Both expressions convey the idea of the speed changing unpredictably and significantly, either rising and falling sharply or in an unsteady manner.
"Fluctuates wildly" emphasizes the large and sudden changes in speed.
"Varies erratically" highlights the unpredictable and irregular nature of the speed changes.
Choose the one that best fits the context or the tone you want to convey.