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Your problem is consistent with an erratic throttle position sensor signal.
It can be directly monitored by backprobing the center wire of the TPS (located on the opposite side of the throttle shaft from the cable controls). A voltage of between .5 and .9 should be seen at idle (close-throttle).
But that's just the beginning. That tells you that (for one thing) I've told you to probe the correct wire. The next need is for the voltage to remain constant when the throttle angle isn't changing.
A simple increase in voltage of 0.06 volt will cause the powertrain control module (PCM) to plug in an idle-up program.
When the throttle shaft angle is seen to increase, it's assumed by the PCM that your right foot is doing the damage. It then institutes a program that supports the liklihood that you would like to go for a ride. It increases engine speed and ignition timing advance.
Once going down the road, the transmission will institute a situation called torque converter lock-up. This happens at speeds above 40 mph and reduces parasitic losses through the torque converter. It's almost a fifth gear.
When the throttle is moved slightly, either up or down, the lockup feature is abandoned. This will cause a temporary rpm increase, due to the sudden nature of the lost lockup function.
Should the reason for the lockup be only electronic (bad information from the TPS), the engine rpm increase will seem odd. But the trans controls are reacting to something not apparent to the driver... the TPS signal.
As a simple test, start the engine and allow it to stabilize at idle.
Tap on the TPS body or wiggle the connector (located on the vehicle left side of the throttle body) and see if you can induce a change in idle speed. A DVOM used to monitor the TPS output is a more positive solution, but simply having a change registered by tapping/ wiggling the TPS connector is usually enough.