When my car (89 firebird w/ tbi) is off, the throttle is nice and smooth, no sticking at all. When the car is running, if the pedal is not depressed then gets depressed, it is somewhat hard to move at first, then moves smoothly once I get it moving. It won't move until I apply a certain amount of pressure to the pedal, then it will move and rev due to being smoother after that point. Any idea what's causing it to have to be pressed harder when it's in the non-depressed position while running?

 

You have the additional forces on the throttle plate (and shaft) imparted by vacuum on an idling engine. Any wear in the throttle body/shaft may cause binding under this additional pressure.

You may need to take a very close look at throttle plate alignment. Any interference should be adjusted out. Chances are, the plates haven't moved, but it can happen, and at least bears inspection.

You need to verify that the throttle cable itself is not failing and binding in the outer sheath.

Proper adjustment of the minimum air position/throttle stop screw may solve or at least alleviate some of this tendency to vacuum the plates closed.

A complete cleaning of the throttle body would be a good start, then adjustment ot the minimum air position and TPS.