I just tried to undervolt and adjust to turbo all-core 3.7G through ThrottleStop.
It did shorten the period of stuttering but still there's lots of room to improve.
A stronger adapter seems still the only way currently to solve this issue.
My 90W adapater is on the way, will update you all the result.
What problems persist with the 90w adapter? I didn't fully benchmark my system with my 120w adapter, but in msi Afterburner the only limit I reached was a temperature limit instead of the power limit, so I thought the problems were over.
I find 70c in my case.
The original limit is around 60.
Envy 13 with similar hardward configuration has the same problem.
I guess the possible story is the original BIOS sets the temperature limit too low.
The power adapter is insufficient as well.
So only when you implement the two measures together (stronger power adapter and updated BIOS) the problem is able to be solved.