Hey fellows looka here I have a DP Extra that had the same problem. This was a porpoising fool trying to land,at half throttle it flew nice and trim. Inverted required no trim change. But when the throttle was cut it would gain altitude just like a glider in a nice breeze. After looking at and reading every thing I could I found an old article form DP himself about the GB Ultimate and why the stabilizer had 1 degree of positive incidence built in. The lite wing loading and high lift created caused the planes to climb at fast speeds. Then when some down thrust is added and speed increases the engine pulls the plane down, then you need a little up elevator to counter the engine thrust. So when the throttle is cut after it is trimmed at speed it starts to climb since the engine is now not pulling it down. Luckily the stab on the DP Extra is adjustable , It is at 2 degrees positive ( up in the front) the engine is now at 1 degree down and the plane flies great. Now I have to hold back on the elevator to land.
So either adjust your stab or rip it out and make it adjustable.

Hope this helps Kent