Well the engine would not actually run if the true AFR was 20:1 so it sounds more like fresh oxygen is making its way into the exhaust manifold somehow. And for the AFR to change that much I would theorize that maybe the AIR pump is coming on and pushing fresh air into the exhaust when it shouldn't . This usually only runs on cold start to help heat up the catalytic converter and the car is usually in open loop waiting for the cats and O2 to warm up. So if the air pump were coming on while the car was warm and operating in closed loop or would cause a problem such as the one you are describing. However that does not explain the flashing cruise control light. Doesn't that flash when the car is misfiring? Which would then indicate that it is actually really lean....hmmm
If there were an exhaust leak big enough for the AFR to go that lean you would most likely hear it.
I'm curious to see what solution you find as this is quit an odd problem. I'd be interested to see if your aftermarket wideband reports the same thing as the factory wideband.
When the car is reading that lean does it richen back up if you roll into the throttle some?