All good thoughts and I agree, start with the basics.
- Disconnecting the battery. Disconnect both terminals and connect the two leads from the car with a jumper cable. Let sit for 15 minutes. This will drain all capacitors and basically be a hard reboot for the electronics.
(DO NOT do this until the battery is completely disconnected. To be super safe, put something between the cables and the battery so they won't accidentally touch the battery posts...or just remove the battery from truck and set aside)
- Reseating all fuses. As you pull fuses, get your flashlight out and inspect the contacts inside the fuse box. You can replace a fuse, but rusty or damaged contacts are easier to miss.
Another thing you can do as a part of your detective work, grab a flashlight and start looking behind panels, under seats etc. for grime or water spots where it shouldn't be. Also check interior metals (such as seat frames) for rust. These would all be signs of flooding.
You mentioned that you bought used. Are there any aftermarket accessories? (alarm, headunit, remote start, backup camera etc.) Improperly installed accessories are notorious for causing weird electrical issues.
Any evidence of previous modification to the truck? (things like spliced wires, unused T-taps, leftover velcro or double sided tape, broken panel tabs, wires to nowhere, holes in headlight covers) Someone may have cobbjobbed an install or shorted something when pulling it out.
Some other thoughts....
- Does this all happen at one time or does something start first which then causes the rest to occur? For example, you listed the alarm first; does the alarm start to go off and then a second later everything goes nuts? If that's the case, I'd be focusing around the first system that fails.
- Are there any similarities to the instances when this has happened? For example, does it only happen when idling....or does it only happen in the rain....or only when you're accelerating....or only when the radio is on? Take a note of the current status of systems next time it happens and see if you can find any associations to future incidents.
- You mention that it started a few months after you purchased...have you installed or changed anything which may have caused it?
- Also, you mentioned it can last for a few seconds to up to over an hour. What causes it to stop?
