Hi there,
Wish you a 240 tab Welcoming to the Brickboard!

You if come back and tell us what fixes this, you too, can become a Brickster!

I really liked the ending you have about throwing a dart at the car to hit the problem. (:-)
I have a story that follows you line of thinking.
This reminds me of a day when riding a motorcycle, during my high school days with a riding buddy girlfriend on the back.
We were on a city back street and ran our bikes up over a very steep but also a down-sided hill, at night!
Never thought about how steep so It surprised and/or scared all of us.
All the weight, of course, landed on the rear wheel.
We drove on for a few miles and I felt the rear end wiggling and it wasn’t her!
That tire became really low so I made it to a gas station.
We took the tire off the rim to fix the tube. They had a water tub at the station.
There wasn’t a hole in the tube anywhere, I mean nothing!

My buddy suggested that we “stick a knife in it” and fix that hole!
I said, What?
He was serious, but We both ended up laughing and consequently didn’t do that!
Motorcycle tubes were not normally found at your local gas stations in those mid 60’s.
I did change out the valve core though since car tires were plentiful!
It is nice that valve cores are still universal today!
By the way, It never leak down again!

So in your case, it might be electrical, if you have a cruise control, maybe with a glitch? Rare but possible?
Not many ways to get air into an engine other than through the throttle plate!
Were you using the cruise control or happen to leave it on during any of those “road trip stops” you experienced?

I think you probably have kept your eye on that or the mechanics should have check it out!
Depends on how thorough they want to be, these days,
Many of them look for the quickest buck and the shops demand it!
The term “Overhead” has two meanings , financially, intellectually and pride

I believe that on your car the cruise operates the throttle from UNDER the dash!
It’s not the most comfortable place to see things easily.
I want to say it’s just as possible the throttle pedal is being bothered by wiring activated by your feet if you don’t have the felt protection in place over the pedals?
A bad throttle cable can hang open the throttle if one and the same are used from under the dash?
We are talking about the throttle plate being held open about an eighth inch or so, to rev the engine!
Check the throttle pulley for smooth operation
I also would be looking for hanging wires or loose electrical connections above the pedals.

If you don’t have cruise control that still leaves a possible internally frayed cable.

Expect more posts, as the site hasn’t died off yet! Newer cars are not as fault free!

Phil