E-Mo
When I meant pitching down I meant towards the ground (level flat ground). I'm not having any problem with it porpoising and it is not glitchy. The elevator servo is smooth operating and has no play as I have noticed that problem on another plane before and replace the servo for that. The plane heads towards the ground when it is right side up or inverted.

redfox, I did balance the plane empty originally (not doubting the manufacturer) but when this pitching problem was noticed, I moved the cg back to where now it happens to balance on the spar EVEN with the tank about 1/2 full (with about 5 oz in it). So now the plane is even a little more tail heavy than when I set it up originally. So shouldn't that help to remedy this problem?
I like the idea of the bladderless tank at the cg but my problem is that the plane pitches towards the surface of the earth even with the slightly tail heavy plane low on fuel.

KJohn, I'm balancing the plane with 2 fingers on the main spar just outside the center sheeting where you are first able to feel the spar. I did this indoors so no wind effecting it. My engine is a Saito 72 (one that the manufacturer recommends) weighing about 17 oz but I didn't think the weight of the engine mattered here since the plane was balanced with the engine on it. Also this problem occurs at mid to full throttle too.

Thanks y'all for weighing in on this problem.