So just to follow up on this issue and to thank @JuanJ for his advice. The first thing I did was to try to find a vacuum leak with propane using an unlit propane torch. No luck with that technique at any of the vacuum lines. Then I bought some starting fluid and tried again. Still no bueno. I then went down an alternative path and started messing with the throttle cable. I eventually got the engine to not stumble but only by adjusting the cable to where the "idle" rpm was around 1100 or so. Obviously that's a bit high but I left it there and ordered a new throttle cable - just because.
Fast forward more than a month and yesterday I was out driving it for the first time in a while and the high idle at red lights was annoying. So I decided to adjust the cable so that the idle was back to a normal range. My thought was to try again to find a leak and/or convince myself to change the throttle cable. Well I got the idle back to 750 and although it doesn't sound absolutely perfect it's good enough that I can't tell if it's ever idled any smoother. Certainly the tachometer is holding very steady and when I had the issue it was bouncing all over the place and the engine would actually die on me occasionally.
So, not sure what the issue is, or was, but it appears to have corrected itself. Maybe some bad gas?