did test cyp 250 mg/ml around 5x already all without problems. could it be your powder source?
anyway, here's how i do it: i preheat oven to 200 to 250 F. ba, bb, gso is put on the beaker. then i add test cyp powder last. i place the beaker on the heated oven, wait till it dissolves completely around 15-20 minutes. then pass it through a filter,going into vials. i usually bake the vials for 2 hours and 15 minutes, just to make sure that the test cyp is completely dissolved. make sure the oven temp is not too high or the rubber stopper and needle (as vent) will melt, you want to avoid that.
the stovetop method is also good, tried it with no problems but i just find it too much work since water needs to be checked often as to keep it from drying