This is just a guess mind you but you seem to be running small props on this engine and I'm wondering if you are leaning out in the air when the prop unloads which might cause you to deadstick from overheating?

I find it curious that both your engines have the exact same problem which seems to indicate
its somethng you are doing wrong with both.

Try moving to a higher pitch larger diameter prop like a 14X10 and also check all the bolts (head and backplate) for tightness. The only bad running 91FX I have ever seen out of many was missing 2 headbolts for some odd reason.

Other problems with this engine have been "O" ring related or even the size of the casting for the needle being too large - both allowed air bubbles which you seem not to have.

My last suggestion is to send one of your engines to someone here that you can trust to check it out for you and see if they can find a simple solution OR send them back to Hobby Services with a note about what is happenening.