SInce no one with more experience has chimed in, I’ll get it started.
Don’t change the rudder setting. It won’t effect the DT. I’ve had similar problems occur in the past, so I’ll give you the most common cures:
1. The stab isn’t “straight” when DT’ed. Sometimes you can’t tell this is happening because it pops into place when it hits the ground. Do several dry runs on the ground and see if this is the problem.
2. The stab DT angle is too shallow causing the model to go into a stall/spin instead of flat DT.
3. The stab doesn’t “pop” right up, but comes up slow enough to allow the model to stall/spin before fully deployed. Once the model enters the spin, it usually won’t recover, even when the stab comes up fully. Look for friction or sticking points in the system to cause this.
I built a Prima many years ago and didn’t have this problem, so I don’t think it is a design issue.