say it slowwwwww. it's your allllllternaaaaatOR lol
seriously. Last year my car did EXACTLY what you are describing, no codes etc, worked great. I never thought anything of it. Just thought it was the fan turning on. (I drove it all last fall and it was doing it with no cause or understanding why, just shrugged it off as minor)
Brought it out this spring, check engine light, p0340, put a new alternator on, it doesn't surge at all, I drove it a few hours tonight and it's smooth as butta all the time. I'd wager your alternator is slowly going out.
And yes, my car would do it cold as well, as soon as I put my foot down just a little bit to get me moving it would jump/surge and then be fine for a bit and then every now and then it would do it less severely but still noticeable. I doubt it's related to your plug swap, I am just offering some insight because it sounds 100% identical to what I was going through. My car doesn't surge at all now.