does it restart instantly? or do you have to crank a bit to get it to catch?

it does still sound like a tb or iac issue. if so it should reset after you try to restart and start fine.
I would see about cleaning it or replacing the iac motor.

the reason I say this, is under load the air requirments are different than you just blipping or reving. when you travel at highway or at speed, the iac is supposed to open somewhat based on total air load so if you suddenly have to slam on the brakes or whatever it won't kill the engine with the tb shut.

it sounds like it isn't opening enough.
so either it is too dirty(which technically would affect starting idle) or it isn't opening enough due to motor issues or whatever.

I have never looked at the numbers to compare so could be off, but that is what it sounds like to me.