#7 not firing has nothing to do with the sudden change in idling problems. It will miss but should just drop some RPM's at idle, not stop idling at low RPM's. If you pull a spark plug wire off of 1 of 8 cylinders, the engine will continue to run and idle at low RPM's.
My suspicion: It sounds like your idle circuits in the carb are plugged. If so, it would accelerate OK with moderate to hard throttle (on the accelerator pump, mains and power valve), and cruise OK on the main circuits. And running OK for the first few minutes is likely due to the choke being engaged and putting in extra fuel, to cover a plugged idle circuit. The idle circuits here include the transition circuits, which are used for the throttle blips when it dies.
I'd start by checking the float level. Not sure why it would change, so probably not that, but almost everything is effected by from the float level.
