There are a lot of possibilities.
I think the three most likely are:
1. You're pouring the bubble into the pan. This seems to have been your own suspicion, and you seem to have taken what ought to be a sufficient measure to prevent it; i.e., a slow pour. Still, you can do more. Allow the batter to settle for a moment, then pick up the pan and give it a couple of gentle raps on the counter (keep it straight while you're doing it) to even everything out. You don't want to flatten the egg white, but you do want to eliminate any large bubbles.
2. You're using the wrong pan. Have you recently switched to non-stick or have you begun greasing your pan? Chiffon cakes depend on "stick" in order to climb the walls and rise evenly. Non-stick is not helpful here, and seldom your friend generally. Too bad.
3. Your oven is uneven and the cake rises quickest where the oven is hottest. However long you're preheating your oven, if it's less than thirty minutes bump it to thirty or even forty minutes.
Other:
If the problem were insufficient mixing, large clumps of unincorporated egg white would be visible in the batter. Presumably that isn't the case. On the other hand, if you only see mild streaking when you're done, that not only ought to be sufficient mixing, it's ideal folding technique. Under-mixing is unlikely, most people actually fold more than necessary to the point where no unincorporated white at all is visible.
You might make sure that the dry ingredients are thoroughly mixed and sifted to prevent a clump of baking powder from creating a localized gas bubble. But with due respect to Chef Chris, I doubt it. Just sayin' is all. But, what could it hurt?
Hope this helps,
BDL
