Truck engine slows to idle. reduced engine power reads across instrument panel and engine light comes on. turn engine off for a few seconds and engine will run fine for a while.

It appears that you have a bad intermittent connection and those are hard to fix because most of the time, like when you want to work on it, everything is fine! But yes, check the wiring and the switches involved in affected circuits you described.

check the engine OBD2 codes. you may have bad crank shaft or cam shaft sensors. replace them all ....

is your car at an idle when the a/c is engaged it takes power to run this circuit your engine will slow down. most cars have a sensor that will increase you idle when a/c is engaged. this also compensates for the added drain on the alternator

check for codes could be tps sensor

This has nothing to do with the heat on the vehicle .
Reduced Power
The reduced power telltale is turned ON by the cluster when it receives a command from the body control module (BCM). The engine control module (ECM) (L81) engine sends a message to the BCM over the CAN serial data link.
The reduced power telltale is illuminated when the ECM is limiting engine torque in a low traction situation. This telltale is used only for the V6 (L81) engine application.
The reduced power telltale is turned ON by the cluster for 2-3 seconds, when the ignition is turned to RUN, for a visual system check.

Taking the vehicle to a qualified repair shop an having it diagnosed would be your best . Unless you have a factory tech 2 scan tool to read BCM DTC'S - diagnostic trouble code's an have factory diagnostic repair info.
Reduced Engine Power Mode
When the ECM detects a condition with the TAC system, the ECM may enter a reduced engine power mode. Reduced engine power may cause one or more of the following conditions:
• Acceleration limiting--The ECM will continue to use the accelerator pedal for throttle control; however, the vehicle acceleration is limited.
• Limited throttle mode--The ECM will continue to use the accelerator pedal for throttle control; however, the maximum throttle opening is limited.
• Throttle default mode--The ECM will turn off the throttle actuator motor and the throttle will return to the spring loaded default position.
• Forced idle mode--The ECM will perform the following actions:
• Limit engine speed to idle by positioning the throttle position, or by controlling the fuel and spark if the throttle is turned off.
• Ignore the accelerator pedal input.
• Engine shutdown mode--The ECM will disable fuel and de-energize the throttle actuator.

Locate the IP to body harness connector, which may be located in and around the left hand kick panel area or inside the IP. Check to see if there is a water leak into the area and repair as necessary.

hook it up to OBD 2 scan retrieve fault code and go from that point!

Only a solution for the maintenance light problem.

Turn off the engine, leave key in ignition.
Press and hold the Select/Reset button on the instrument panel (the reset button you use when you're clearing your trip mileage) and then turn the ignition switch On to the second position -- not to the start position.
Hold the button until the indicator resets -- approximately 10 seconds).
bikerbc

The above descriptions of the connector are very good. I washed the engine compartment yesterday, and immediately started getting the reduced enginer power code on the CIC, not performance. After reading several posts, I checked the connector on my 2003 2500HD, and found that the connector was not seated well and had gotten water in it. I cleared the water and the reconnected. The cables running from the fuse box to the connector are so short, the connector is in a bind and is pulled off-center. That is why the connector wasn't seated properly. The code went away, and the performance returned. By the way, be sure to do this after pulling the negative cable on your battery.

HI...
THis problem is caused for
Lost of correlation between ACELERATOR PEDAL POSITION
AND THROTTLE ACTUATOR MOTOR.
Because your car has no acelerator cable.
The morer probable is the Trhottle actuator motor come damaged. 90%. 10% the accelerator pedal position.
If you turn of the car it will work OK> for a few time.
But the Service engine soon. will stay on.
Yuo can go to at ANy ADVANCE AUTOPART, AUTOZONE or NAPA AUTOPART. They make a diagnostic free.
If not. I recomend get the car scanned for see exactly why the light come on. and make the right repair procedure.

OK>> I hope this help on your question...

Thank you for use fixya.