Car hesitating or lacking power when accelerating

I got my used Mazda last year in August and after a short while I noticed that it hesitates when accelerating, especially uphill.

So far I have checked and done the following:
1. Changed my spark plugs.
2. Changed my air filter.
3. Changed my fuel pump.
4. Moded my catalytic converter (put straight pipe).
5. Cleaned the throttle position sensor, air flow sensor.
i have also had my car checked on a diagnosis machine twice and no errors have showed up. None of the above solutions have worked

Please help