Another thing to check is the engine's thrust line.

It sounds like you may need some more down thrust.

Flying at full throttle, and trimmed for level flight. Quickly pull back the throttle to minimum and observe the result right after you do that. If the plane climbs breifly, then you have too much down thrust on the engine. If it quikly dives then you have to much up thrust.

What it should do is go straight ahead, and slowly enter a gliding dive.

It sounds like yours will dive quickly.

The incidence and CG also factor into the equation. You may have to fiddle with each to get the plane well trimmed out.

Trim charts such as the one linked below are useful:
http://www.tracyskyliners.com/AirplaneTrimChart.htm