I get 420 a lot, and many times 301 is included. I had been thinking that 420 was due to loose exhaust flange, but i understand that shouldn't cause a miss.
The miss usually comes after long periods of granny driving, so I always assumed it was carbon build up on non-rotating valves. If I get on it through 1st and 2nd, over 4k rpm (rotating the valves), the idle is then smooth.
But now you got me wondering about my #1 injector, as this too could cause 420 cat error.
Anyway, mine is similar in that it comes and goes...I'm not terribly worried about it, but yeh, would be nice to solve.