The warmer the truck gets, the worse the idle gets

Could be a problem with the throttle body. If it was ever cleaned, that may have caused the problem. The throttle body on these vehicles should NEVER be cleaned.

Also, look for air leaks in the intake system. All air entering the engine should go through the Mass Air Flow sensor. Look especially in the PCV line, vacuum line to the transmission, and intake manifold gaskets.

The fuel pressure regulator should not have much effect on idling. With a fuel pressure gauge connected, removing the line should cause the pressure to rise to about 45-48 psi. This is the only way to test the function of the regulator. The idle may rise slightly because of the air entering the vacuum line.

If the ignition coil was causing a bad idle, it would be more pronounced under load. A bad spark at idle becomes a bigger problem under load.

EGR is a possibility. You can check this with the self-test. During self test, the PCM opens and closes the EGR to verify operation while monitoring the EGR backpressure sensor for correct readings.

The most sensitive part is the throttle body. This is designed to never need cleaning because the throttle plate is sealed when the throttle is closed, and idle air is directed through a calibrated hole in the plate to maintain a minimum idle speed. The PCM then operates the ISC valve to obtain the correct idle speed and to compensate for engine loads such as A/C, power steering, and transmission being in gear. A sticky ISC will give you a fluctuating idle as will a throttle body that has been cleaned.

ISC is the Idle Speed Control motor. Lotated near the throttle body, and has a 2-wire connector plugged into it. This controls idle speed by bypassing air around the throttle plate. It is controlled by the computer. This can stick and cause idle quality problems.