Geistsware is correct. You really need to add a bit of downthrust. Downthrust is used to minimize trim changes when you change power. If the model tends to nose up when you add power, and drop the nose when you cut power, you need to add some downthrust. A mix won't really solve the problem. A correct thrustline will.

To add downthrust, just loosen the engine on its mount, add a small washer under the mount on the mounting bolt on both the left and right side. This will angle the engine down a bit. Tighten the bolts. Fly the model and get it trimmed for level flight at your usual throttle setting. Now play with the throttle to see how it behaves. You may have to add another washer or two.

You can also add downthrust by inserting washers between the top of the mount at the upper mounting screws. Again, a little at a time.
Washers at the top of the mount will tilt the engine less than washers under the mounting lugs of the engine, so you may find a combination of the two will get you so close that the amount of pitch change with throttle change is minimized.