It sounds like your Vehicle Speed Sensor (VSS) went out. When it goes out, the car doesn't know how fast the car is going, and therefore doesn't know when to shift.
There is a second, far worse possibility; your tranny just puked. When the differential goes out the engine will free-rev cos it basically slips into neutral. Dry driving at 45-50 and slipping it into neutral without lifting off the throttle... see? engine shoots up.
There are only a few things that will cause uncommanded RPM increase: unintended TB manipulation (Throttle cable pulling itself) is one. There is one common way that this happens: the front motor mount breaks! When that happens, the engine will buck up, pulling on the throttle cable, revving the engine. When it happened to me, the cruise control cable snapped and locked itself in a position that kept the engine apprx. at 4500 rpms, bouncing off the rev limiter in neutral. I lifted off the throttle, engine kept GOING! that was scary.
Second way that happens is unintended gear change. This will happen when the tranny slips out of gear (into neutral) or rapidly hunts for a gear (1! no, 4! no, 2! no, 1!). Either one (or both) of these can be caused by a bad VSS. The car can get an approximate idea of what its speed is based on gear selection (tranny), RPM, and TPS position (and cover sheet).
This can also be caused by a faulty shift solenoid, burned up clutches/bands, a blown-up differential/other hard part, or low/no fluid.
So, the first thing I would do is check the tranny fluid. Make sure you do this at warm idle; the level will read low if the car's off. If it's at the right level, and red(ish), I would suspect the VSS. This will usually throw a code, so CHECK YOUR CODES! After that, I'd find the local AAMCO...
edit: omg 899 posts!
