Dual-servo elevator linkage geometry
Andy,
Try the diagram again with both servo arms parallel to the elevator at neutral position. You may need to move a servo to make this happen.
The model may be tucking due to one wing snapping.
I have a spreadsheet which will calculate and diagram your servo/arm/surface movements to great detail. I can email if you like. I forget where I got it. It's rccalc95.xls
Are you selecting matched servos for the elevator pair? (speed and throw)