You aren't going to hit that limit safely ever, if you have a two 18650 batty mod. The board will SUPPORT the wattage but no battery is going to safely achieve it. Not even close. It seems ridiculous to me to use that type of board unless your mod supports the 27-- series, and even then, you'd have to get really high mAh rated batteries.
As to your resistance fluctuations, if you've checked the build, well, it may be the SS, it does vary a SMALL amount at times, but it shouldn't really be enough to cause ANY difficulties in the right TCR mode. You may want to watch a youtube video or two on the device, to get more of a handle of its capabilities. Etc.
Also, I like a cool vape, but I would consider 167 watts to be a scorching vape. Use what your batteries can handle and forget the "upper limit." Either that, or get a 27-- dual mod, or a 3 battery 18650 mod.
I don't think your equipment has issues, I think it's more your EXPECTATION of what it "should" do that it literally, "can't do. Also, I think watching a few tutorials on mod and whatever tank you are using might be helpful. It's just my opinion as always.
However, it is FACT that you will not reach 167 watts with two 18650 batteries. Your board might "pretend" to do it but basically you'd be overstressing your batteries with a single puff.
I agree, if the mod doesn't do what you "want" there is an issue, but it's really marketing of those insanely higher and higher "boards" that don't advertise that achieving them is technically IMPOSSIBLE.
Best of luck,
Anna