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Ok lets start with some basics. What engine and trans do you have, gas or diesel? Is the trans fluid level full or is it low and smell burnt? The check engine light on or od lamp flashing on/off? Have you had a diagnostic check for codes done with a scan tool?
Ok P0603 indicates you have a performace chip installed in the vehicle or aftermarket tuner. First thing I would do if you have the tuner is revert the vehicle back to stock and see if you have the same issue with the trans intermittently start slipping. Sounds like an electronic concern based on your description
dOES IT HAVE A TRANSMISSION TEMPERATURE GAUGE AND IT INDICATING ITS RUNNING HOT? The tow haul light doesnt flash and no codes for the trans fluid temp sensor? this is the most common thing I would see that can cause this type of concern and you replace the tft sensor and internaL transmission wiring harness to resolve it, when it sees the trans fluid is too hot it puts it into a default mode. Monitor the tft sensor when it acts up and see if its running too hot the transmission