Rather than saying that "the lights don't dim at all", it would be more helpful to post the battery voltage. It makes a difference if the battery voltage is 12.9 or 13.4 or 14.1.
Since you are certain the charging system is in tip top shape, we can move on to more tests.
The fact of the matter is there are many possible causes for the reported symptom. A short list includes:
- bad/loose battery or battery cable
- bad/loose battery ground
- Bad alternator
- Bad/loose main battery junction box terminal.
- bad ignition switch
- bad CCRM
- For applications with a 2 speed fuel pump return system, bad HI/LOW speed fuel pump relay or ballast resister.
- bad PCM
- about a 100 possible motor causes including ignition, fuel, vacuum leaks, MAF, and so on.
Trouble shooting a problem like this is a process. In my experience it's best to start based upon the symptom and the easiest/most common causes first. I tend to try tests instead of "parts changing". Especially the tests that are fairly easy to preform.
An easy way to confirm stable voltage is by using an "add a fuse". Use the add-a-fuse on the I/P #18. Attach a volt-ohm meter (VOM) and use this to monitor the voltage while driving. What this test should do is to rule in/out what the voltage is doing at the time of the problem.
I/P #18, this is the pilot duty circuit from the ignition switch. The output of I/P #18 goes to the CCRM. So knowing what is happening at this point may tell us which way to look next (down stream towards the CCRM or to the ignition switch).
An ODB2 scanner and a set of wiring diagrams may make short work of this problem. The ODB2 scanner could be used to monitor MAF values to see if there is a sudden drop BEFORE the motor dies. There are tons of other tests that it's possible to do with an ODB2 scanner.