Lpc2000

Hi,
we are programming a lpc2294 and have problems with the ARM/THUMB mode.
We'd like to use the mixed mode (arm & thumb mode) to save time
loading the programs from a 16-Bit flash memory. Unfortunately the
program get stucked if an interrupts occurs when we are using the
mixed mode. With the arm mode everything works fine.
Can someone tell us how we can also work in the thumb mode?
Thank you!
Matthias