[W]hat are the root historical causes of the current Republican Party dysfunction? When did things start going wrong? Was it 1993?I don't think we really know, but I think there are two or three reasonable theories.
One is that it's demand-driven. Essentially, there's a very good market that responds strongly to The Crazy. They'll donate for it, they'll buy products of it, they'll listen to or watch shows about it. It's not a huge market, but it's big enough that it creates seriously goofy incentives for Republican politicians and other party leaders -- there's a lot of money in being Rush Limbaugh, so much so that third-rate talentless hacks can make a very good living as pale imitations of what he does.
A second theory is more elite-driven, and is based more or less on the concept of "learning" -- that people tend to copy those things that they perceive of as having worked. Alas for the Republicans, the two big winners over a half-century were...Richard Nixon and Newt Gingrich. So if you emulate those two, you're going to wind up with a much worse type of politics than if you emulate, oh, FDR.
But wait -- isn't there also Ronald Reagan? Yup, there is. But for a variety of reasons, Reagan's pragmatism hasn't been much of a model for the GOP. The big thing that they took from Reagan, alas, was that detailed policy knowledge was a net-negative: thus George W. Bush. There is an impulse to find a Reagan -- thus the movement to draft Fred Thompson in 2008 -- but that's about it. Instead, they've invented an ideologue Reagan who never compromised his pure conservative principles, something that Reagan made easy to do because his particular political (and personal) genius was precisely an ability to constantly believe in his own purity regardless of what he had actually done.
As far as when they become dysfunctional, I don't know...I don't think I'd put a clear date on it (although the Gingrich defeat of the Bush budget summit agreement is surely a very big marker). Some of it goes back, perhaps even before Nixon; some of it appears to be much more recent.