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!

UR QIY FEWMPIQR PHT RHS VFB NYFT QIMO WR JNMYBT M FW MWANYOOYT DYNR ZYEE THBY RHSN OHESQMHB LYR MO BNNFNPUUNHUI QIMO EMQQEY VIFEEYBPY ZFO BHQ QHH IFNT ZFO MQ