DMX512 by any chance?
If you really need a continous stream at 250k baud then I think you need to some careful planning (and I guess that IS what you're doing with these test so thumbs up!).
At 250k baud you've only got 40us to do stuff Before you need to feed the next byte the UART (well you CAN wait up to 80us since there'a basically a single byte buffer) between each byte.