Joined: Mar. 2005
The term "Darwinism" seems to mean whatever aspect of evolution theory a particular speaker happens to disagree with. With that shifting of meaning from one speaker to another, it might be better to avoid using that term.
As for the idea of increased mutation rate on population drop, it sounds plausible to me (though I'm not a biologist).
But as to whether or not it's inconsistant with current theory (inconsistency with Darwin's original theory is irrelevant), I don't see any reason to think it's contrary to the currently accepted theory.
About the notion that evolution theory claims only gradual changes over an entire species - that wasn't even what Darwin said; it was the scientists that followed later that added that bit, and scientists today certainly don't think all evolution was gradual (some of it, but not all, maybe not even most).