Yes, this probably is an engine trim problem. Trim it to fly straight and level at about 1/3-1/2 throttle, or whatever is normally cruise speed for you. Now give it full throttle. If it pitches up, then angle the thrustline down a little bit. If it pitches down, give it less down thrust/a little up thrust. If it veers off to the right, give it a little left thrust. If it goes to the left (this is more likely), give it some right thrust. Remember, a little bit of yawing to the left is normal because of P-factor.

Remember to make all of these changes gradually. Many people get into trouble by changing things drastically.