You need to add some up thrust. You should be able to fly along at full power and cut the throttle (touching nothing else) and it should keep going without suddeny climbing or diving.

Obviously when you cut power it will start to descend, but it should be gradual. Also try flying at half throttle, straight and level, then go to foll throttle. It should not change suddenly, just go straight ahead.

This is common with the Cap, the low wing/high tail and thrust make them perform well but not so easy to trim.

It also sounds a bit tail heavy.

TF