Just to clarify, you're talking about the older model ETS 150/3, and not the newer ETS-EC 150/3?
In either case what you're describing is not normal, and should warrant a service call. But the underlying problem might be different depending upon the model. The newer EC's have electronic speed control, which modulates the speed depending upon surface contact and a variety of other factors, and which every so often can behave weirdly, like slowing down for no apparent reason. For the older ETS, the speed is fully manually and mechanically controlled, and so the fix may be as simple as making sure the speed dial is actually interlocking with the speed control mechanism.