Thumb change alone

Hello ,

I'm using Keil µVision on a LPC2387.

Here's this strange behaviour :
From time to time, when going through the code __disable_irq();, the Thumb mode is cancelled, and then later it falls into an exception.

Do you have any idea about this ?

When it occurs it looks like an interrupt happened, (The debugger shows the Interrupt registers) but I could continue doing a step by step trace in the same code (One step from two is skipped)

extern QUEUE_TRANSFER entry_que(QUEUE_HANDLE channel, unsigned char *ptBuffer, QUEUE_TRANSFER Counter, unsigned char ucCallType, QUEUE_HANDLE DriverID)
{
    QUEQUE *ptQUEQue;
    QUEUE_TRANSFER rtn_val = 0;

    uDisable_Interrupt();                                                // disable all interrupts

    switch ( ucCallType ) {

...See printscreen at tech.voila.net/pb_thumb.jpg...