Actually, facetious comments aside, the Goths weren't a fad, they were a 3rd/4th century tribe. You can read about them here.
http://en.wikipedia.org/wiki/Goths
So, you can see that the Goths were a Germanic tribe of very early history, and the Anglo-Saxons were dated a few centuries later, (8th/9th century). This fits in, as Guinevere explained earlier, with what Tolkien was doing giving the earlier Northmen Gothic names and the people of Rohan, hundreds of years later, Anglo-Saxon ones.
Vikings were also a Germanic culture, and thus related, but their geographical and linguistic origins were rather different, they came from Scandinavia whereas the Goths and the later Anglo-Saxons were from mainland Europe.