Excuse me I think I badly asked my request!
I know you can overclock with R button but only at 100, 120, 133, 150, 166 Mhz.
My GP32 doesn't support 166 Mhz so I put on 150 but sometimes with 15 fps all images can not be read (rarely fortunately!).
So I think 156 Mhz should be better for me but how if we can ?