I have a problem, I use a queue in an ISR, which starts a task with the highest priority. When I give  the interrupt for free, the ISR starts some milliseconds after the event, which is to late! After 3 or 4 events everything is ok, the ISR starts after some  microseconds. I use a LPC2119. The interrupt has the second highest priority. The highest is the timer for the os tick. Maybe someone can help me?  

Do you mean the ISR starts some milliseconds after some event – or that the task start some milliseconds after the ISR? If the former, then are you keeping interrupts disabled for too long anywhere in your code?