So I took more readings with the system turned up high and got the same readings.

I also re-checked my ground and sanded under it.

I also added a wire from the neg. bat terminal to solid ground.

Just for the hell of it, I ran a test wire from the battery to the remote terminal on the amp(with the lead to the HU disconnected). Got the exact same results with that setup.

Is the amp hot to the touch when it shuts down? Is it going into protect or completely shutting off (protection light on or nothing at all when this happens)? Is there any audible engine noise or crackling coming through the speakers with the engine running prior to it shutting down?

the amp is not hot when it shuts down.

On this amp there is no protect light, from what I can tell there is just a red light inside the amp that turns on to indicate the amp is on. When the sub cuts out, this light stays on.

Here's what the light looks like:

http://www.mtx.com/caraudio/products/flash/ampsRTP251D_slideshow.cfm

there is no noise when it shuts down, the rest of the speakers play smoothly.

If it was shutting down under protect you would likely know, because the light would change color or flash or something to indicate that it has. Did you run your 4 gauge power directly from the battery positive terminal, or from the secondary fuse panel under the hood (if it has one, I know my Four Runner used to which had a positive threaded stud for connection of accessories). Are you by chance using a set of ground loop isolators inline of your RCA's?

I ran the 4 gauge right from the + terminal on the bat.

My RCA's are running direct from the HU.

I'm gonna throw my amp in from the camaro & see how that goes (kenwood 250)

to me it sounds like a bad rca or a bad speaker/wire connection that is causing your deck to cut out..... and vibes from the engine are straw that breaks the camel's back... so to speak....

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SIC Audio

Check the terminals of the speaker, check the speakers wires of the deck, try with the deck just besides the amp. What kind of box do you have??