Weird Firing problem

I have an A-5 + R-T.

My marker was doing "PUUUU......" when I press the trigger.

So I took it to an airsmith, he said it was the spring and replaced it.

Then it seemed fine, but when I went to the field to play it.

It goes off like before, "PUUUUUUUU..........".

The gas leaks by a lot, and the response trigger is not functioning right.

But I know it's not the R-T problem.

Anyone else has any idea of what this is?  Appreciate your help, thank you.

Sorry I am pretty new at this.

Where should I look for the sear?

The gas tube is fine, there is no leak... I've checked for leak many times but don't think there is a leak.

I could be wrong with RT too.

So you suggest that I replace it?

If this is not the case, can there be any other kind of problem?

O-ring? Not oiled well? Which section of the gun I need to double-check on?

Can the valve be the problem too?

Thanks!!

Forgot to add,

If I press the trigger all the way, it works most of time.

If I intend to press the trigger half way to simulate the automatic RT provides, that kind of problem occurs.

I did a leak check, there doesn't seemed to be any leaking.

The gun fires well like I described, if I press trigger all the way.

If I press it a little bit, then it will fire like continously abnormally.

Like "POOM....OM....OM", which wastes gas and not firing any ball.

Gah.... so painful.