Car revs without pressing accelerator pedal

Hi all,
Sorry to post another question here. I am getting mad with my second hand car. You better never buy a second hand car like me. :(

Today afternoon, I drove my car for two and half miles. Suddenly, I felt something was not right, the car revved up and down and up and down itself without my foot. When I stopped the car for checking, the revolution was up and down between 900-1500 at idling. It was happened for about five minutes. I checked the accelerator pedal for stuck but nothing. Then, I drove the car to my home at very low speed and now the car is running normally.

But, I feel very unsafe and do not know what it is. I want to lean what is the problem before bringing the car to a mechanics for checking, as they will charge me very costly here in Ireland. I do not have much money :(.

Can you guess what problem caused my car revving up and down and up and down itself without my foot?

Thanks.