To try and solve your issue, do you see a noticeable nose up or nose down when you gun the throttle or when you back off suddenly? If so, you have a thrust line issue which is pretty simple to correct. But if you don't see any pitch change tied to throttle changes but rather a gradual increase of altitude when the plane is going fast and a decrease when it's going slow, you may have a CG or an incidence issue. It also matters what kind of plane you are flying. A flat bottom wing is always going to be speed sensitive, at least as long as you keep a forward enough CG to have any degree of stability. Symmetrical wing planes usually can be trimmed to be very close to perfect in a wide speed range and hold their pitch in all throttle positions.