unequale acceleration at starting (civic cx 1999)

Dear Honda experts,

I have a honda civic cx 1999 hatchback and when the engine is cold
(first 5 kilometers) I sometime have a very strange problem. It's like
if the engine is not answering when I accelerate. Here is an example:
I start the car (everything is fine). I push the pedal once, the
engine answer promptly and I push it more, now nothing happen like if I
didn't push the pedal, but I keep the pedal push and after 2-3 sec I
got the acceleration. It will happen 1-3 times and after the car work
fine.

Any idea of what is the problem?

Thank you very much