Sputtering/Misfire?

I've already put close to 1,000 miles on the "new" old 4R.
I took it on a pretty hard run this weekend - 4 wheel trail over a mountain pass, that drops down into the Great Sand Dunes National Park.
4WD worked great! Over 300 miles round trip. Maintained 65mph+ on the hard top, at around 100+ degrees, and at elevations between 8-9000 ft.
Again, I'm still brand new to 4R ownership, and I want to learn what I can, rather than just taking it to a shop every time something comes up
I'm noticing more and more of a "sputter" coming out of the exhaust, and it's more noticeable around the O2 sensor. Is this a misfire? The previous owner had just replaced the head gaskets, the timing belt, right before I bought it, and said the plugs, dist, etc., where only about a year old... but that it had been sitting for a while. I've been running 91 octane and Lucas Injector Cleaner since I bought it.
What are the thoughts on where I should start looking first?