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Did the check engine light come on?
Does the light flash when it's at low RPMs?
So other than the low rpm issue, do you have any issues with acceleration or high speeds?
The possibilities that you listed usually trigger a check engine light to come on. I am surprised that it has not come on yet.
To me it sounds like the Idle Air Control Valve (IACV) is not allow air to by pass into the chambers which is causing a misfire.
Remove the IACV and spray it down with carb cleaner. Then wipe it using a clean cloth. Put the valve back and see if this improves the issue. If there is a change in the operation, however it does not solve the issue, the valve may need to be replaced.
The valve is mounted on the throttle body and looks like this:
Also check if there is any wear/damage where the arrow is pointing on the picture.
We would have to check the spark plug gaps (since they got changed recently). We would also have to clean/test the mass air flow sensor and possibly do a spark test to see if the plugs are firing correctly up to specs.