hmmm...

i would ask the following - when it has popped does the epoxy in any way seem to be still soft?   if that's not the case, and your epoxy is curing, a common problem is a starved joint.   assuming you do not have an epoxy chemistry problem, try the following approach.

first, make sure you have a clean-dry parrallel surface.  sanding with 60 grit sandpaper should be just fine.  second, coat each side of the joint to be scarfed with unthickened epoxy and let it cure until it is just tacky/pasticky.  this will seal the open grain of the wood so when you glue with the mustardy cabosil mixture, the epoxy will not be drawn off by the open grain potentially leaving you with a dry joint.   third, after the seal coat is mostly cured, glue the joint with mustard-thickened cabosil epoxy.  do not clamp so hard that every bit of epoxy is squeezed out. 

give that a try....it should work

howard

   I have made sure that there is ni sighn og being tacky i have alsi taken it down to bear wood and glooed ontop of old epoxy. It has never separated from the wood there is always still epoxy on both sides. I will try the way that you said and go from there thank you

   Scarfs are 8:1 (or more) tapered "ramps/wedges", right? Not a square butt joint?

-don't let the straight epoxy precoat cure at all (no need, risky to let it cure too far); add thickened runny paste and align and lightly clamp. 

   I suspect there is some type of contamination on the wood. I would wipe the wood down with denatured alcohol and allow to dry after sanding and before your pre-coat. On the sheer clamps you don't need much weight/ clamping pressure to hold the joints together.