Lpc2000

I'm trying to write to the PINSEL2 register, to ensure that it has 0's
at bit 2 and 3, but if I try to write to the register, the JTAG
connection drops, and it seems as if the MCU enter some other mode,
since I can't restore the connection whitout having to do a power cycle. 
Has anyone experienced this?
And how can I write to the PINSEL2 register, whitout having this problem?

/Søren