Hello Welcome to JA!
I will post on Q@A
I would look at the TPS/AP
Throttle position sensor
Your having a chocking and losing power at 12 to 13 rpm's, and it sounds like a consistent problem at those rpms.
The TPS/AP is located on the throttle pedal
there is 3 screws that attach's this to the pedal and one wire connection.
Many times times this will give a problem as you are describing not showing a code or showing any other problem
here is general information
General Throttle Information (Automotive Limiting Speed Governor)
The correct TPS/AP Sensor counts for DDEC engines at idle should be 64 - 205 counts.?
If you disconnect the wire harness and use a OHM meter you can see if the voltage is sooth from 0 to 100 if at any point while pushing on the pedal and the voltage is not sooth and has a low or flat spot then you know this is the problem.
Let me know if i can help more!