Gotta ask, why aren't you just using normal braided steel lines, like .008's or .012's?

Spiderwire makes a fishing line called Stealth.  It comes in 6 lb (.005 in) and 8 lb (.007 in), as well as larger diameters and strengths.  It is my go to fishing line for striped bass.  It is actually a braid that has a low friction coating on it.  It would eliminate the stretch you are experiencing but also reduce the tangles because of the coating.  It might be worth a try.  You can order it on-line directly for Spiderwire.

I finally posted a flight video here;

https://www.youtube.com/watch?v=rDfcimUYEP8&feature=youtu.be

   I improved my ability to fly this little plane enough to
land a second place at Brodaks 1.2A contest.
  Tim Stagg won with his Tsunami. Good on him! There
is no shame in coming second to Tim.
   The quality of the planes and competitors was amazing.
( Scott Richlen photo )

  This video was in early May and the circle was still soggy from
the winter melt.  I was rusty but the Cox TD wasn't.

  You can hear the engine going rich on the outside turns.
A slightly leaner needle solved this and I didn't have this
issue at Brodaks.  I don't think I did, I was busy steering .

   The plane flies well but it is very light and it takes concentration
to keep it in the groove.  It turns faster on the outsides
probably because of the drag of the landing gear.

  I am very happy with the TD.
Paul Smith was correct about just sleeving the venturi
to improve the draw. That was the only modification.
I'm using one head gasket, a high compression head,
a fine thread NVA and Riches Brew 10%  50/50 Synth/ Castor.
  I'll do a post about it later.

Cheers! - K.