I had this problem once on my old 91 4runner. The cable was frayed where it went through the firewall, I had to replace it. I didn't know what it was for a long time, but I would do just like you, tap the throttle. Finally one day it frayed so bad that the throttle would stick open however far I had pushed the pedal and I would have to kill the ignition so it didn't rev way up. Then stop and get under the hood and pull the cable back to idle position. That was a fun trip home. That is when I found out it was the cable.

1997 T100 extracab, 3.4L, 5-speed, 169K, 4x4. Love the truck, never buy anything but toyota, I tried it once, most miserable six months of my life.