Oh dear, I guess you have cracked the two caesar cryptos...
This one is more difficult. Although a simple substitution is easily cracked...
Again the characters are limited to A-Z... But I think I can come up with a 256 version again.

Enjoy!

SX YCF WIOZQCYX QVA XVR MWD PFWA YCZG OX NPZFDA Z WO ZOKPFGGFA BFPX LFII AVDF XVRP GVIRYZVD EFX ZG DPPWPQSSPVSC YCZG IZYYIF MCWIIFDQF LWG DVY YVV CWPA LWG ZY