2003 Toyota Tacoma Engine stalling at idle, hot or cold.

About six months ago, after a brief stop at a store, the truck would occasionally refuse to start unless the accelerator were depressed, and then it would run fine. Gradually, this problem became worse and more prevalent. Now, the truck refuses to idle most of the time, and is now very difficult to start even when cold.

All vacuum-hoses appear to be in good condition and the fuel filter is new. I listened to the idle-air control valve with a stethoscope while the ignition was being turned on, and heard only a buzzing, not the click of a moving part. I do not know what to expect to hear. The fuel-pressure-regulator is one of my suspicions, but I am not sure. I do not know what to try next. What are your thoughts?

Thank you for your time,

Chris