Need help improving aircraft stability

I am having problems with 2 aircraft I'm building, and they both have longitudinal stability problems. They tend to do a backwards flip right from the runway during the takeoff roll, or if I work hard enough to get them airborne sometimes there is a tendency to do a sudden "tuck" and point down sharply. I am using canards and delta wing one one while using a diamond wing configuration on another. I am trying not to use hoz. stabilizers on the tail, only a vert. stab with rudder. I am making sure the craft is not too heavy and that the CG is ahead of the center of lift. What are other things I can do to improve longitudinal stability?