Now call me an idiot (ok you will!) but why shoud the idle air control cause a car to cut out when it is at 45mph and your 'having trouble keeping it going' i.e. probably foot to the floor.

The MPG, well that could be ok, with towing a trailer and probably an autobox, not good but ok...

From what you describe it sounds like the previous problem was the car overboosting anf the fuel cut off comming into play...but others may disagree (if so i bet it went like a rocket!)

What you are describing soundsl ike one of a number of possibles...

1. Crap in the fuel system causing blockages (had it happen to me...but i would have expected it to hit faster than it did on that long trip..)

2 Fuel pump relay, never had it myself but they are known to give probs, are easily soldered and cheap (ish) to buy.

3. Ignition...this sounds like a more likely source to me...I would check your leads, sparks, rotor & dizzy cap...and replace them all unless ytou have doen so recently...also think about doing the coil...coils break down in new, interesting and damn frustrating ways! I have seen them go so that the issues only show with the engine under load (like up a hill....)

4. Throtle body...clean it -

5. well lots of nasty stuff like ecu's etc....but do the ones above first...

of course you may already have done it all already...

i share your pain...if i could work out why my brick drinkls a litre of oil every 150 miles, but only on long runs and without smoke i would be a happy man - ps tried a new turbo too...ouch!