Runaway Engine

98' Duratec, while driving today at a steady 40mph, RPM's around 1,500 I placed auto. trans. into neutral (foot OFF of accelerator) and the engine started reving higher, the RPM's went up to around 2,600 and stayed there until I applied the brake (still in neutral) as I was approaching a red light. Then the RPM's started coming down to idle position. Now, I know this isnt' normal and to make sure I'm not crazy I came home and did the same procedure to my 2000 Explorer and when I placed it into neutral the RPM's and engine speed dropped to a normal idle position. Anyone have any ideas as to why my car is doing this? Maybe a bad sensor or vac. leak? No codes stored. Only other driveability problem is bad hesitation from a dead stop in traffic. Maybe related?

A guy went to the doctor and told him that when he raised his arm above his head it hurt. The doctor told him not to do that.

The computer is probably not understanding what's happening when you put the car in neutral at 40 mph, it knows it's going 40 mph so it figures the engine should have a load on it and not be at idle, so it bumps the power to an appropriate setting to try and compensate. That'd be my guess.