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2012 Mazda5 with Hard Shifting (4th ->3rd)

Hey all, so I have a 2012 Mazda 5 (Automatic) and it's my wife's daily to shuttle the kids around. For the past 2 years we've had this weird issue when accelerating. Specifically, when driving around 40 mph the car is in 4th gear, but if I open the throttle the car hesitates for a split second, the car jerks (feels like the engine is jumping out) then the car speeds up in 3rd gear.

The car has 110k on the odometer, and all maintenance is complete (filters, oil change etc..).

Since the problem came up, I've:

Changed the transmission fluid (no change)
Changed spark plugs (no change)
Changed ignition wires (no change)
Changed battery (was due anyway, but no change to driving)
No check engine light, and there aren't any active codes coming up nor pending ones.

I suspect carbon buildup (not sure how..) because:
My car had trouble starting and after cleaning out the throttle body and replacing the MAF sensor the car started right up with no issues. Look how dirty the throttle body was! [pic]

I poured in Techron concentrate on the last two fill ups. The first one didn't really do much (10oz?) but the second one (20oz) seems to have calmed the issue. There is a slight jerk you can feel in the pedal but not as a passenger if that makes sense. Both times I poured it in when the tank was empty and right before filling up with some more Chevron gas. I gave it a spirited drive immediately afterwards for 20-30 miles (not high speed, just high RPMs on all gears).

I might try another bottle just to be 100% certain.

The only other thing I can think of is there is some mix setting in the ECU or whatever that's stuck and isn't adjusting. I thought I reset the ECU already when the battery was out, but could there be some other procedure I'm missing? What's weird is the problem almost always happens in 4th to 3rd gear. None of the other gears have this problem. I guess it could also be the transmission control module (TCM) but I haven't heard about issues on the 5 with this, just the 3.

FWIW I took the car in to the dealer for the serpentine belt and they didn't see any issues with the car. Should I just take it back to them and show them on a drive? My only worry is there could be something super easy that I haven't thought of and paying through the nose.

My wife drives it most of the time and she has a light foot anyway, so she never runs into this issue. Maybe I just drive it too hard?
2009 Mazda 5 GT
Does it still do the hard shift when you're in manu-matic mode and tell it go from 4th to 3rd?
I think you're on the right track with some kind of buildup, that throttle body looked really bad. Have you also cleaned out your MAF sensor?
Make sure all your motor mounts and transmission mount are solid. They seem to be a common failure point and would definitely make a hard shift feel worse.
2010 Mazda 5 Sport
I agree, on checking the motor/transmission mounts.

Based on the description of the symptoms, I'm not feeling that it's anything engine related.

Transmission or mechanical.