Bin the shockcord; it was never a good system, and there are better systems available nowadays.
Replace with two separate control lines, not elastic but there is no need to go overboard with high tech non-stretch materials such as dyeema; standard polyester is fine, around 4 mm. One control line is an uphaul, led to a standard clamcleat on the tiller; the other is a downhaul, led to a CL257 auto-release clamcleat, also on the tiller.
Adjust the tension on the pivot bolt so that the blade moves as freely as possible vertically, but with no sideways sloppiness; that is likely to be quite a fine adjustment, which can be found only by trial and error.
Hope this helps,
Oliver