The problem Smoothpass notes, can really be a bugger.
Typically -it is a simple high speed stall.
the fix for ONLY that problem is to reduce pitch to eliminate the stall.
But -on models there are a couple of other gotchas
If the model is constantly yawing - (hard to see)- any sudden pitch makes lift change from wing panel to wing panel -and -over she goes.
try various rudder trim
Also if the elevators actually move at different rates -- this will do the same thing - make the model yaw -then pitch.
The best "FIX"- is to make the airframe as stiff and accurate as possible
BUT also, add no weight
The higher wingloadings typically operate closer to stall angles -so the problem is worsened
just keep pecking away at it .
Looks like a lot of good answers posted here - just start with the easiest fixes first-