Fuel delivery was also my first thought. I had very similar symptoms due to old varnish melting off the sides of the gas tank and plugging up the pump -- would start and idle fine, and drive all day under light throttle, but as it for enough gas to go up a steep hill or anything else that required more throttle for more than 15 seconds, and it would starve.
Ignition problems would show up as soon as you stepped on the gas hard. Fuel problems only show up when you step on the gas hard long enough for the pump not to keep up with the reserve in the float bowls.
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