'87 FJ mystery issue update:
Replaced coils to no effect. Plugs show heavy black soot coating. Bike runs perfectly when cold. As the bike continues to run and warm up, the idle begins to die down. This seems to happen faster if the bike is actively being ridden versus being left to idle. Eventually, the bike will not idle, and runs poorly: poor running and stumbling at low-med rpm, and won't rev past ~5000. Requires significant throttle input. Ignition module is functioning properly: swapping with another unit had no effect. Unplugging the vac advance line on the ignition and inducing a vacuum leak increased the idle temporarily, but died back down after a minute or so.
I should also add that this is a sudden problem. The bike has run fantastically since I got it. This just showed up suddenly one day.