Ok, I would start with changing the spark plugs. It seems the issue is only under load. This would eliminate a bunch of things, especially with no check engine light.
If the plugs don't look bad, or make a difference, then the next thing you can check at home would be the fuel pressure. You would have to rent or borrow a gauge, but honestly, there isn't a lot that will cause this problem and not turn on the check engine light.
Weak ignition and low fuel pressure are among the most common issues. The interval on the plugs on this is 30k I believe so if they haven't been done, it might just be a simple tune up that will resolve the problem for you.
The fuel treatment also a good idea, even if it's not the injectors, then it's good for them and the valves anyway.
Now, it is possible there is a problem with the mass air flow sensor or one of the O2 sensors, however a scan tool is really needed to watch their inputs to the computer to be sure.
So I have to suggest stuff you can likely try at home first, then take it from there.
Hope this helps!
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