MY CAR CONDITION IS WORSE

Hello guys,
I have a Toyota Opa, model of 2000/1
it has almost 9 month since i started
using.

The thing is that;
Few month ago (around last year
2014) it statted behaving weird it was
smoking - Black smoke.

I took the car toachenic and he
checked it and he advised Me that I
should change the plugs., with ought
"hesitate.", I agreed with him and the
plug were changed.

To My great surprise, the car was
again well, no smoke though it wasn't
going as it used to go before the
smoke issue.

2month later, the same problem
repeated and this time caused the
car's engine to go " off" itself.

What would it be guys.?!

I really love my car, and it pains Me
when I see it not doing well especially
in performance on the road as it was
b4! When I press the "over drive
button" it went as if it is on "2" or "L"
mode with some strange sounds from
the engine.

Please let Me know.