Thanks for that suggestion; I *thought* it seemed similar to heat throttling I'd experienced before, on an old desktop build, but didn't want to think that this old clunker was getting to that state.
It does seem to be a temperature issue; I've temporarily resolved it by moving the unit onto a stand for better airflow, but she's still running a bit too hot. I'm not overly concerned about it... because, well, I can't be (older machine, so part life expectancy isn't much of a factor anymore, and I simply can't afford to have a professional do maintenance on it. Assuming they even would).
Thanks for the tip.