What have you done to get the engine running? The possible explanations for this are legion, from not properly tuning the high speed (should be adjusted to maximum RPM and then turned to the rich side a few clicks), to an improperly adjusted low end (rich low end loading up and then sputtering on throttle up), possibly air leaks in the fuel lines, foaming in the fuel tank, dirt or other foreign matter in the high speed needle, worn cylinder, bad glow plug, bad fuel....

Tell us exactly what you have (engine, fuel, glow plug, prop make and pitch), what you do to start, run, and tune the engine.

I know it gets repeated in every thread in this forum, but it applies. Take the plane and engine to a local club and find someone with experience that can help you. Otherwise you'll end up getting frustrated.

Good Luck!