Just for the halibut, throw some lead up front and see if it helps....or take some careful measurements and post it here for someone to calculate the tail volume coefficient and CG on this model. Maybe there's a misprint or mis-calculation on the orig. kit plan. Who knows!

Data needed:
Wingspan, root & tip chords, leading edge sweep offset
H/Stab span, root & tip chords, leading edge sweep offset
Distance from wing L.E. to stab L.E.

There's an Excel program download somewhere on this forum that does all the nitty gritty stuff for you if you have the above measurements in hand to plug in.

Good luck