So this has happened with different planes, different radios, different servos and always after several flights with no issues.

The only thing I can think of is that you are having an abrupt CG change to aft during flight making the airplane too sensitive in pitch.

Is it possible that you have balanced several of your planes, for example, by adding weight on the face of the firewall or motor mount somewhere such that your routine attachment method is eventually failing after a number of flights (tiny screws that are backing out due to vibration?) Do you have airborne packs installed for balance and they are shifting in flight (ie. velcro adhesive breaking free or strapping etc.?)

Again, I can't think of anything other than a CG shift that would cause sudden and abrupt in-flight changes to pitch sensitivity; and since it is ending in a crash, the shift would most likely be to the tail.

I'm so sorry to hear about your experience with this and I'm interested to understand the resolution. I hope you resolve it!

Merry Christmas RCU buddies!

Tom