As for increasing the clock speed in GP2Xpectrum you need to load a rom and then press select.
Near the bottom there should be a section that says "FS0, Contention off (155 Mhz)", select it and press X until you see (175 Mhz), return to the game and see if the flickering stops.
If not then I'm sorry but I cannot help you more as this is more of a F200 problem.