97 4Runner SR5 Auto with 171k miles.

Ok!, It has been probably discussed multiple times, but I haven't seen any one experienced P300 and P306 simultaneously... and more interestingly....

1. a month ago experienced loud engine (but running fine) then, one day popped the "CheckEngine".
2. Immediately, checked underneath and found "busted" muffler.
3. Welded Magnaflow Stainless steel muffler.
4. Interesting is that shortly later, "CheckEngine" light disappeared automatically.
5. But more interesting is after muffler replacement, car often shakes rough but only when the weather is warm. Not sure if it is just coincidence...
6. It has been getting worse and worse.
7. Finally, threw "CheckEngine" light a few days ago. Blinking Blinking crazy.... But engine never stalled....
8. Checked the code, P300 and P306.
9. Today, it is cool and rainy outside, Car has been running fine for whole day....

Will start with Ign Coil and Spark plug... but
If any other idea, it will be very helpful...

Thank you