Yup! Too bad as it looks very cool. An alternative you might want to try would be to add diagonals to the existing ones to make triangular shaped "true" warren truss type structures. Find some way to test the resistance to twisting before and after adding them (with the covering removed so you're comparing apples to apples) so you can evaluate whether the fix is likely to be adequate without adding the sheeting. If the increase in resistance after versus before is significant you might want to recover and try a flight or two. Doing so "will" add some torsional resistance over the existing structure. If it is still questionable go ahead and add the sheeting which should be a sure bet.
It's not a torsion thing. It's the fuselage bending from side to side, which actuates the elevator -- the further the pushrod pivot points are from the fuselage, the worse the bending. Mark Scarborough found this out a few years ago with a profile that misbehaved in the wind. I think he actually had a camera mounted on the wing watching the tail as he flew, at one point.
He put strakes on the fuselage from TE to tail, and the flight performance was vastly improved. There's a rather long thread in the rules section about it, because the width across the strakes was well over the regulation 3/4" for a profile fuselage.
Motorman: do it up with really thin fiberglass sheets back there. They won't be as rigid as carbon fiber, but they'll be translucent!