Your idling problem is most likely either the IAC or the vacuum leak. If the leak is bad enough, the IAC will not be able to properly meter air past the throttle place causing the idle speed to constantly overshoot or undershoot.
Since you didn't previously specify that you have an SVT, I had qualified my previous response. In any event, the engine will run just fine on 87 octane as will almost any current or recent Ford engine. You might loose a bit of performance on the higher compression engines, but it will still run fine unless there are other problems that the octane is masking. My Navigator has run perfectly on 87 for a long time. I only put higher octane in it when I know I will be towing my boat.
Steve
