Throttle body & plate needs cleaned.  Deposits build up which make it
"sticky".

There are spray cleaners available at the auto parts store for this
purpose....just make sure you get one which is not harmful to sensors,
etc.  You'll have to pull the big hose from the air filter box which
attaches to the throttle body (just loosen the screw on the hose clamp,
and wiggle it off).  At that point, you'll be able to look into the
throttle body and see the plate.  Follow the instructions on the can
from there, and put the air intake back in place when you're done.

I've had to do this several times over the lifetime of my 99 Camry.  The
first time, I took it to the dealer.  The other two times I did it
myself.  Not a big deal either way.