Could someone explain returnless systems?

[demonpixel]

good suggestion, mark. i don't think that's it though because just now i opened the throttle wide open (car is off), and the secondaries came down all the way just fine. they only get stuck when the engine is running. VOODOO

if it was the gasket it'd get stuck whether the car was on or off right?

[70bird]

Voodoo indeed! If it was an interference problem, it seems to me it wouldn't matter if the motor is running or not. Is there a stepper motor of some kind on the pass. side? Probably not. How about the TPS possibly hanging up?

Maybe try this, shut the motor off when this happens and see if the secondaries close by themselves. If not you can rule out a vacuum related issue, and get back to looking for a mech. interference. Maybe you could disconnect the new linkage, start the motor and rev it while opening the secondaries manualy to see if the same problem occurs. If the problem persists, you can rule out a problem with the linkage. At the same time you could check (with the motor off) the movement of the secondaries independently to feel for any sticking points.

You stated that when this happens you have to close the secondaries by hand. Do you mean that you are pushing on the secondary shaft to do this? Or can you just push on the primary shaft to do the same thing?

As Andrew said, sounds like you have a defective throttle body.
Hopefuly you can exchange it!
Good luck