No, the solenoid (along with the converter & transmission pump) is working fine as evidenced by your test. It seems that the problem lies in control of the solenoid. It's not being properly commanded.
The paper clip test also discounts the brake switch as being a problem. That's because the +12V to the solenoid passes through the brake switch. The paper clip is merely providing a path to ground for the solenoid. Since you successfuly locked the converter by grounding it, the +12V side is fine.
The ECM activates the solenoid by grounding it as well. Something is affecting the ECM's ability to lock your converter consistently. At least you've narrowed down the problem.
Could be VSS signal, 3rd/4th gear pressure switch signals, TPS signal, coolant temp sensor reading, ECM, PROM. Do you have access to a scan tool? Something like an OTC Monitor 2000/4000 would be helpful as it provides a visual indicator as to when the ECM commands TCC apply.