This is known as "punctuated equilibrium", and is generally accepted as the standard evolutionary model.
Actually no it isn't, most evolutionary biologists I know are less than impressed with the idea. Most evolutionary biologists would probably tell you that the rate of evolution varies greatly and that apparent evolutionary stasis (the hallmark of punctuated equilibrium) is probably just a wrinkle in the fossil record.
Agreed, and what's this in the summary about evolution progressing "smoothly". I belive that the late S.J. Gould demonstrated that it actually proceeds in spurts or maybe it was Dawkins. Regardless of who's idea it was it has been known for quite a while that evolution is not a nice smmoth curve.
The evidence for Gould's ideas concerning punctuated equilibrium has never been entirely convincing to me. PE has also been referred to by those in the gradualist camp as 'evolution by jerks' on the odd occasion....
"When the going gets tough, the tough get empirical." -- Jon Carroll