2000 Mitsubishi Eclipse P0141 and P0120

I have a 2000 Mitsubishi that I took to the shop last week. The code read P0141 and P0120. They told me to replace a new TPS sensor (code, P0120). Today it has a new TPS sensor and it still doing the same thing. I start the vehicle and put my feet pressing down the peddle. When I let go the peddle, the engine shuts off. But if I keep pressing and hold the gas peddle, it doesn't shut off only if I let go. Code P0141 has not been fixed yet. Some people told me it's the Idle air control valve, but my mechanic didn't say or mentioned that is the cause since he did some diagnose. So I assume its not the idle air control valve.