The model trim and balance along with the wing and stabilizer surfaces and engine thrustline angles all come together. And then there are misalignments and warps to consider. Without going over the whole airplane and checking EVERYTHING it's impossible to say what the problem is.

I'd start with a very complete measurement of the model to confirm wing and tail surface alignment angles to within one degree and do the same with the engine. Check for any warps in the wing and control surface misalignments as well.

In an otherwise properly aligned model with 2 or 3 degrees of downthrust already in place I would say that your balance point is too far forward. This results in requireing more angle between the wing and tail. When that angle is quite large it emphasises the sensitivity to power to make the model climb. In THAT case moving the balance point back a little and re-trimming the elevator to compensate will reduce the model's sensitivity to climb strongly under power.

But it's in your own best intrest to carefully check the model over first. It's bad enough with a kit or ARF but with a scratch built model who knows what angles were built into it.