It sounds nose heavy. It might sound contradictory, but here's what happens: You trim it at 1/2 throttle and everything is fine, but in order to achieve that when it is nose heavy, you have to use more up trim than you would use if the CG were further back. Therefore, when you advance the throttle, it pulls up.

In general, a nose heavy airplane will have a less sensitive elevator, but also a bigger trim change in response to throttle.

Jim