UART problems with LPC2129

Hi,

I am using hyper terminal to read and write data via UART0 with LPC2129.

in my code if I run the reading task alone it works,
if i run the writing task alone also works.
If i use writing and reading tasks (write each 1sec, read each 200ms) only one of them works....(the one whish is added first in the scheduler.)

Any ideas plz?