I agree on the "iffy" connection... I had the very same "vanagon syndrome"
I did all I have to but it's still there and then one day while replacing
the air filter I discover that the AFM harness is very tight.. the previous
owner put a zip tie too close to the main harness and that give too much
tension.. so when the engine vibrate at around 3K rpm or going over a bump,
I got the hiccup...