In my expierence, when the 2.5L motors go, they just go. They don't let you know about it. I've had a few friends with over 150K HARD miles on they 2.5's and they suffered massive failure.
As for the high idle check your TPS. I have had problems with my 2.5 and it either was electrical or was cleared up with an engine treatment. The lucas oil and fuel additives in my 2.5 did cause it to idle high. I quit using them and it returned to normal.
Try running a can of B&G 44K through your fuel. It's expensive and tough to find (some dealerships carry it). But it works wonders.