Engine shutting off at idle, dash lights flickering, and engine starts on it's own!

[jack.duno]

I have an 08 Yaris hatch, automatic with 60,000 km (~37,000 mi. and half of that was put on in the last 3 years.)

The car was just in for service at a trusted mechanic for an ABS code. The right rear wheel speed sensor was replaced. New rotors and drums were also put on since the car had been sitting for 6 months prior.

Today I got in the car after getting it back from the shop. I do nothing but get in and start the vehicle. 60 seconds of idling in park later and there is a thin metallic clunk (like the sound of a heat shield falling off). The engine tone immediately changes, the RPM drops to 400-500, and all the dash lights go out (gauge backlighting included). I quickly shut the car off and get the technician back outside to see this.

I go to start the car to show the tech and the starter engages, but the engine does not run. I try again. The car starts and runs for 10 seconds and then shuts off on it's own. The tech brings the car back into the service bay and this is where things get really strange.

The car is now back in the service bay and the tech is sitting in the car with the key in the ignition and the car randomly starts all on it's own. It does this multiple times over the course of a minute, occasionally shutting itself off. What's interesting is the starter would sometimes still be engaged even after the engine was running, almost as if someone was holding the key down in the "START" position. At one point the lead mechanic came out and asked if it had a remote starter or any ignition modifications... (which it does not.)

The tech who brought the car back in had also said that their expensive scan tool suddenly stopped communicating with the car, despite it working just fine two days ago when they pulled the ABS code. It's worth noting that they also had a cheapo scan tool on-hand which WAS able to talk to the ECU without issue. (There were no codes stored)

The car stopped acting up and I carefully drove it back home, no issues whatsoever while driving. I park in the driveway and shut the engine off. And then I start it again. That's when this happened:
https://www.youtube.com/watch?v=4Sl4...ature=youtu.be
Right as I start recording, the engine idles down and the dash goes out again, just like it did the first time in mechanic's parking lot, only this time there was no metallic noise. 15 seconds later it happens again; the engine drops in RPM and the dash lights go blank. I tilt the camera down because I'm getting ready to tap the gas just to see what happens, but the car returns to normal before I actually do anything. Over the course of that video, I do nothing but hold the camera. There is also another car driving by during the video, that engine sound is not coming from the Yaris.

It is worth noting that while the gauge lights go out, the gauges are still functioning and displaying the correct RPM. Everything seems secure under the hood, and jiggling the key and banging all around on the dashboard does not trigger issues. This is all very weird. The lead mechanic had said in 38 years, he's never seen a car that just starts on it's own. The clunk that was heard when this problem began in the parking lot makes it seem like a mechanical issue, but everything else points towards electrical! There is no engine light on, and there is no battery light. The battery was replaced 2 years ago with a store-bought, brand new battery. Like I said before, the car had been sitting for 6 months (over the spring/summer), but it was started regularly and had been driving and running fine for the past week. This problem only started after getting it back from the shop. I have tried to replicate the issues after shooting that video, but have not been able to.

I've looked up and down the internet and I'm not finding anyone with this issue. I'm looking for anyone who has had similar problems or any suggestions whatsoever. Thanks in advance!