Thank you for your help in advance…

I have problem with my car.
Since yesterday, the engine stops when it was stopped after slowly driven.
At first, it stopped in front of the gate of my apartment.
I slowed down and stopped behind another car and then I stepped on the accelerator but it went backward (I was on slight slope).
So I puzzled and pushed brake, and I found out that my car was turned off.
I restarted and slowly drove.
But it turned off again when I stopped the car just few feet away from the gate.
And today, it happend in front of the garage and cross road.
It didn’t happened when I drove fast and there’s no problem when I start up.

I really have no idea.
What is wrong with my car?

Just in case…
it’s 1998 mazda, 11*,*** miles, timing belt and front brake pad were replaced few months ago…