"#1 on my list, Bad Ignition box."

100%

It's not an orange box by chance is it?

The last ECU I had go bad would cut out and fire back up when going down the hwy. Made for one heck of a backfire because of it!

You mentioned ground, make sure the ECU has a good, solid ground. On my drivers, I'll run a braided ground strap from the block to the ECU and use a nut and bolt with a star washer to fasten it to the unit.

Do you curse a lot below 70 mph? Must be rough going thru school zones.......