HCW, I, and I am sure many others, feel your pain.
As I mentioned earlier, you should be getting some type of fault on the J0 screen, Obviously, you'll be getting the "Impact Error". Are you getting any others? I would assume; perhaps you could ask your tech; that you are getting a "front eye fault". If your mid limit switch and wiring are truly working, I'd say it is the front tbar eye.
I have been told by many that these intermittent errors, though they are tough to diagnose, usually are wiring issues. Maybe you could run one of the spare wires from the J0 all of the way to the front eye amp? Test the amp by swapping with a know good amp. Actually swapping the the front amp for the rear amp, will let you know if the issue is the amp. You'll start having the issue on the rear side of the vehicle. If the amp is good, replace the front eyes(don't splice). New eyes(must match the brand of the amp) straight to the amp base. Make sure all of the wires in the base are tight and not corroded. If you have another amp base, swap it out when you swap the amp. You might have Carlo Gavazzi amps, so you'll need Gavazzi eyes.
Also, the fact that you were having issues with a corvette(low profile) initially, leads me to thing you have a front eye, front eye wiring, or amp issue.
I hope this helps.