Easy to diagnose--fly your plane at full throttle. When you let off throttle, does the nose drop? If it does, you need to add down thrust to your engine. If it doesn't, then it is probably an incidence problem. Remember, though, trainers tend to climb with power anyway, and sigs are really known for having a lot of positive incidence in their trainers--especially in the kadet series (speaking from experience of my Dad's senior arf). Anyway, either fix will probably solve your problem.