I'm not completely sure what exactly you are asking.
Can you provide a better description of what exactly you mean by
"20 deg. cross on"
Is the wind causing you to "pinch up" while you are waiting for the waves?
This causes you to lose speed and you are having a problem getting back up to
speed to accelerate onto the wave face when over powered?
I also don't understand why you would be doing better when less powered and having problems when more powered to overpowered.
Are you sure you are sheeting out as you bear off onto the wave face?
We all tend to oversheet a bit when overpowered, so maybe you are reluctant to open up your rig to the full power of the higher winds as you turn downwind to catch the wave.
Give us a little better idea of the wind direction vs the wave direction and perhaps we can give you some ideas to try the next time this happens.
Hope this helps,