2000 Toyota Camry after

I am having this problem for almost one year:
symptom:

when car is cooling down (means when just start running in the morning or after car rest for few hours and start), it does not have problem described below

It happened when the car is getting warm up (after driving for around half an hour), when I stop the car at the traffic light or stop sign then try to go forward, it always hesitate to start, feel a stuck, has to press pedal hard, feeling like trying to separate two magnet

There is one time, I stop at a right turn corner, my steering wheel is at right turn position, then press pedal, the pedal completely stuck, which means I can not give gas, however, the car can move a little bit, I pull it over the side and stop engine restart car, then it can move again.

it bothers me a lot every day, I tried a mechanic, he open the front lid and put some oil at one place and said it fixed, but it does not at all

if any one experience same problem, please kindly help. Thanks in advance