Yep - both problems sound TPS-related. The TPS will cause the trans to
lock/unlock (it's probably the converter locking/unlocking, not actually
shifting between 3 and 4), and will also cause the computer to chase the
idle... TPS spikes to a higher voltage, and the PCM immediately increases
the amount of fuel, anticipating more air. But, since the throttle plates
haven't really moved, the idle goes rich, dropping the RPMs, then the PCM
cuts the fuel, going lean, bringing the RPMs up. Lather, rinse, repeat.
To the OP: the Throttle Position Sensor is bolted to the side of the
throttle body, underneath the air intake. It's the sensor on the opposite
side of the throttle cable linkage. Do you know how to back-probe an
electrical device, and check it's voltage output (preferably with an analog
multi-meter)? If so, you can diagnose the sensor yourself pretty easily.
If not, you could have a shop do it, or you could gamble the $40-or-so for a
new part, and swap it out, hoping that it's really the problem.
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