I've been building and testing an tailed delta wing'd RC aircraft, but it has seemed that the plane is extremely pitch happy and tends to somersault when at high speed, is there some type of easy fix? More counterweight at the back?
pitch happy and tends to somersault when at high speed
The old saw is to move the weight, but the most time honored approach is to glide it unpowered first. Make weight adjustments here.
This helps sort out stability and trim issues of the design.
Now add power. Thrust angle must be correct or you wind up with a plane that is very difficult to trim.
I had a similar experience with a Delta Ray RTF model. If the thrust line was not correct, the plane had to be re-trimmed significantly for low speed landing approach.
You may find that adjusting your thrust angle to push the nose down when power is applied works well. Models commonly have a thrust down angle of 3 degrees or more.
If you are building an aircraft that will fly through a wide range of speeds, relaxing static stability can help. Keep in mind that this will no longer be a "trainer", and expertise in piloting will be needed.
This is accomplished by moving weight back and reducing aerodynamic tail downforce. Remember, moving weight back reduces longitudinal stability as well, meaning larger horizontal and vertical stabilizers may be required.