i have my 4g93 mitsubishi engine, hesitate to accelerate and engine fail to start what can be the problem?

That’s a vague problem. But most commonly it’s an exhaust issue. Check your air flow systems first. If your air filter is bad. Or If your catalytic converter is bad. You’ll know if the Converter is bad cause it will glow red or stink. Or if no pressure comes out of your tailpipe

Hello, I replaced the gear box as many faults were there in existing one. But new one also not work properly. while driving the gears get slipping? Can you explain why this?

Check the clutches of the gear