Brian, are you running steering AND rudder? With both of those, veering off should not be an issue......and a jet needs those in my opinion (lost a plane mainly because it had no rudder......)

Apart from that, i see no reason why it should not take off on 800ft (well cut) grass. Could you describe how it accellerates? Does it just accellerate to a point and then keep going at that speed, or does it keep getting faster? If it maintains a certain speed, the drag from the grass is too great.....mow it shorter then .
If it does keep accellerating and should be around flying speed: Do you have a cheater hole below the fuse? If you do, chances are it will suck itself to the ground...had that on my sabre before conversion....only fix there was to pull the power on rotation/liftoff....not a wise thing, but worked (most of the time even without damage to the plane *g*).
CG and throws are all set up correctly?
Regards
Hank