Lpc2000

I have a strange problem.

I am using a vectored IRQ on UART1. Sometimes I boot the application
and it works fine. Other times I boot it and it locks up on the IRQ
vector handler, that is it keeps re-entering the IRQ handler and never
process the 'main loop'.

On Entry to the IRQ handler, VICIRQStatus=0x80, indicating Uart1, but
the U1LSR register is 0 (most of the time, regular int's also occur).
I am writing 0 to the VICVectAddr at the end of the routine, and like
I said, sometimes it works fine.

Is there some condition where it is possible to have a U1 interuppt
when U1LSR=0?