SOLVED. 70s/80s fantasy/alternate-history where world still at middle ages level thanks to a supernatural horde/intervention. [s]

[Lee]

In the mid 80s I read a fantasy book set on an Earth where technological and cultural progress had been stopped at the Middle Ages level, thanks to a series of supernatural invasions at key points and locations at set intervals (maybe every 70 or 100 years?). Perhaps starting in the Dark Ages, a horde of goblin or troll like creatures would roll over a region like a barbarian invasion, and in so doing set society back, especially in Europe and the Middle East. One invasion occured just as the crusades were about to start, blocking Europe's access. Another in Spain & Portugal, before Columbus was to sail. Next is Germany, so the reformation is quashed. Other invasions may have been in China, crippling their development. So anyway, its now the present day (about 1980 or so) and our characters are in a medieval style England, just as another invasion starts there. As you might expect, there's a quest to get to the bottom of this cycle, and at the end whatever magical/evil force is causing this is somehow stopped (don't recall how). Anyone know this book?

[Christine]

Could it be an Eric Flint book? He writes fantasy mixed with alternate history. Usually in series, I think?

[Ann aka Iftcan]

The Fellowship of the Talisman Boy it's nice to be the first one to come up with the title of a forgotten book.

[Lee]

Ann aka Iftcan - yes, that's the one! Thanks for that.
Thanks to Christine for your guess, I may look into that author.