As you have already found, with the TPS disconnected, the reading rises on its own. This is expected.
Knowing that, you can now see that with the TPS connected, if you experience the same, there is likely a problem with the wiring to the sensor, or the sensor is faulty in its own right.
Other than that, there is the slim posibility the input on your box may be faulty, but I think that would be exceptional.
Check at the TPS connector, the ground is good, there is volts between the ground and 5v. if those are good, check the resistance of the sensor its self changes with hrottle movement.