Expert help on Vitz '99

Salam to all. I wish I had found this forum earlier! Already have an Vitz '99 (silver, a / t, no CNG) owner for about one year and all it's a great experience and very good value for money!

Except for one small issue, which started about 3 months ago. And that's what I was looking when I found PW.COM.

So here goes ...

Some times, when accelerating from a standstill (eg a signal) the car would not respond. It would be 5 kmh max 2-3 seconds ... During this time when I press the pedal more, the car starts kind of choking ... and after 2-3 seconds everything is back to normal. Once it begins to accelerate, it is almost nothing and nobody can say that there is something wrong with the car!

Now this problem used to occur very rarely. It was more noticeable when there is any load (eg a / c was more than 2 ppl sitting or car was on a climbing road).

During the last three months, since I first saw this, the problem is slowly more and more noticeable. Now every time I start from a dead stop at a light climbing road, I see the 2-3 second delay in response. Even the choking is now so serious that during this time the brakes hard (maybe not enough power is produced).

I've already changed the spark plugs and got the throttle body cleaned (do not know if the mechanic did well). The "check engine" light is normal - not lit when the car starts. According to the muffler guy and a mechanic, the problem is not with the cat and converter. Now I need the car to the mechanic because he will clean the sensor (probably the O2 sensor on the intake) and see if it solves the problem. Otherwise, he says that the sensor should be replaced.

Any suggestions?

Plus, now I think about it, seems the most significant decline came after the last service / wash done. There is also a very small rattling noise, which is jointly developed with this problem. Hi octane also tried, but even that does not change the light rattling noise.

Will appreciate the help. Thank you.