Sven
OK, yes, altitude will influence the wing performance a huge amount. We fly only 50m above sea level. CROW is going to reduce the wings lift, so I don’t see that helping you here.
Are you sure all your radio conditions are the same, you don’t have something changing at full flap? The way you seem to be describing the handling sounds more than stalling, normally even a tail heavy airframe is stable until pitched in someway by a control deflection
Balance was checked with everything in position, except wings removed and landing fuel in the main tank ( not full of fuel)
Dave