I just recently finished building a Quickie 500 type airplane that was kitted to be a "Sunday flyer", tricycle gear, canopy etc... Basically it is an old school low wing Quickie made back in the mid eighties that I found on the hobby shop shelf.
After messing around with the configuration a bit, I ended up building it just like a Quickie racer, tail dragger, and a OS 40 FX with a Mac tuned pipe I had laying around doing nothing, looped up over the top side of the fuse.

I took it out Sunday and it leapt into the air, no lack of power on this one. The only problem I seem to be having with it is when it approaches its top speed; the tail (pitch) will start to violently pitch up and down uncontrollably. It will stop as soon as I back of the throttle a little bit. Now I would assume some kind of flutter problem with the elevator, but the kicker is that this same action will occur when you take the A/C vertical and just when it is about to lose forward speed and you roll it over, the tail will start to pitch in a wild oscillation again.

Any Idea what my problem could be? Is it a control problem, servo or aerodynamic issue? Any input would be appreciated.