I've never seen this on a Yale- only on Toyotas, but you never know- try adjusting the ignition timing when the engine starts to break up- could be a ignitor going bad.

On Toyotas I've found that when the ignition electronics start to go bad it affects the ignition timing. Usually it's during the starting cycle that the issue comes to light- unit won't start- extremely advancing the timing while cranking will get the engine to start- then retarding the timing to normal specs after starting keeps the engine running good.

Just a thought- the chatter could be from the ignition timing changing on it's own- try checking timing with a timing light while the issue is happening- keep me posted