Wotcha think of this one?  Tiny, tough, fast, stunt/combat trainer. 

I think I'm in love.  I want one

A kit may soon be available.  Flight tests first.

there's a story behind its concept; I'll relate it sometime.

A few details:  Uses the droopy diamond airfoil; low aspect 18" span wing makes it about impossible to fold it up; extra tall fuselage right at the weakest point (wing LE); upper MM beam extends all the way back over the stab.  nothing's unbreakable but this should make it a challenge anyhow. 

I thought that tail was kinda odd.  Of course, I'm not so sure about the airfoil, either.

What are you planning to change first?

Before you do that, can you fix up the leading edge of the airfoil somewhat?  All those sharp edges where you want air to be flowing smoothly would cause turbulence where it doesn't belong, which certainly could account for "sluggish and twitchy at the same time".

If you were up to hot-wire cutting foam, I'd suggest taping on some prosthetic leading edge material with shipping (or plain old scotch) tape.  Then if it didn't work, you could take it off easily enough.

OTOH, the stab location could be doing the same thing to you -- it looks like the stab wants to duct the air in a specific direction, which may make elevator response funny.  But I don't carry a CFD machine around in my head, so I could be totally off base.

Tim, the Droopy Diamond airfoil has worked extremely well in several other applications, both stunt and combat (slow speed and high).  It's not "just" a diamond airfoil:  The diamond-shaped ribs are deliberately spaced far apart to let the covering sag between them, producing a more standard airfoil in cross section between the ribs.  Ribs jutting up then serve as "fences" helping direct the airflow.  I don't think that's causing any problems on Stumpy since my other designs using it are very successful.  see pix.

I get it now -- interesting.  That means you could do a droopy stretched hex, too, or other "droopy" shapes.

I think you could move the stab back and/or down without losing much, if any, of the looks of the plane.

Well, l've gone the other way...I cut the tail out this AM and reinstalled it down in line with the wing.  Just to see what difference it would make.  Haven't tried it yet.  Moving the tail up, I guess you mean near the top of the rudder--might be another "trial" to try.

the controls are plainly too sensitive, that's for sure.  I foolishly used a Perfect 1/2A alum. BC I had laying around because it was already strung with leadouts, etc.  Reminded me why I started using 3" BCs in 1/2As in the first place.  And the CG is too far forward I'm pretty sure.  Lead on the tail will remedy that, on this one anyhow.

Try tying the stab into the wing trailing edge (I'd do it with scraps and shipping tape).  The gap there is probably making odd things happen.

I really don't see anything basic with the design that should cause problems.  I think you can find a solution to whatever ails it.