Spikes while observing my obd2 scan tool

This sounds like an issue with the ECM, or the harness between the DLC and the ECM. I've never heard of a fuel trim going anywhere near that -93.8. Even a -20 would be indicative of a problem.

Going back to first principals, we have a few possibilites:

The data for the IAT flows from the sensor, over the harness, to the ECM, then back through the harness to the DLC, to the scan tool.

Fuel trims are controlled by the ECM and (duh) reported back through the DLC to the scan tool.

Spark advance is controlled by the ECM and reported in the same way as the other items.

Common components here are the scan tool, the DLC, harness from the DLC to the ECM, and the ECM. (Assuming that you don't have a big section of your engine harness that is missing a bunch of insulation and shorting out somewhere, which would drive the ECM nuts.)

Make sure that the pins in the DLC aren't pushed back into the connector, which may cause an intermittent communication and/or power issue for the scanner. This happens more than you might expect, typically due to some over-zealous probing of the DLC (and pushing pins back into the connector with a multimeter probe).

Examine the harness for any obvious signs of damage. Damaged insulation, exposed wires, breaks, wires that aren't connected to anything, etc. Engine wiring harnesses are pretty resilient, but sometimes things get messed up.

If the connector and the harness are in good shape, then the next item on the list is the ECM. A flaky ECM can cause all of these problems, and may still do a halfway decent job of running the engine. I suspect this is the root of the issue.

A bad ECM and/or a faulty harness would also account for your tool disconnecting from the ECM.

Another possibility is a bad ECM ground. This could be due (again) to a faulty wiring harness, or even potentially to a failing battery. Batteries have an average life expectancy of 3-5 years, though some have gone much longer.

You can get a used replacement ECM pretty cheaply, but:

• It should be from a vehicle that has the same engine AND transmission as your truck, and preferably with the same options. (Though, most of the "options" will be handled by the BCM rather than the ECM, so there is some wiggle room here.)

• It will likely have to be re-flashed with your truck's VIN. Dealers can do this, but I'm not sure how much they'll charge. Note that some dealers may refuse to flash a used ECM that did not come from them. Call around before buying anything. Small shops that have a Tech II can do this as well, but it requires a subscription to GM's online service.

• • Even without flashing a replacement ECM, you would be able to connect your scanner and see data from the car. The IAT readout, for instance, will still report roughly ambient temperature without the engine running. That would give you some feedback about whether the ECM is the problem.

• You'll have to reprogram your keys. On most GM vehicles, this can be done by you in your driveway in about 30 minutes.

Good luck!