Lpc2000

I am using a Nohau JTAG ICE connected to the second port on a LPC2106.
The emulator does not make a reliable connection to the JTAG unit on the
chip.

On my design, I have DBGSEL pulled to ground.  Right after reset, I
execute the following code segment:

  ldr  r0,=0x55400000
  ldr  r1,=0xE002C004
  str  r0,[r1]

On my board, I connect the 20-pin JTAG header to the LPC2106 like this:

  NTRST (3)	-> P0.27/TRC0/TRST (8)
  TDI (5)	-> P0.30/TRC3/TDI  (15)
  TMS (7)	-> P0.28/TRC1/TMS (9)
  TCK (9)	-> P0.29/TRC2/TCK (10)
  RTCK (11)	-> RTCK (26)
  TDO (13)	-> P0.31/EXTIN0/TDO (16)
  NRST (15)	-> RST (5)

Right after enabling the secondary JTAG port, I have a code loop that
toggles an I/O pin.  I can see this happening, and then when I run the
emulator, the toggling stops.  It doesn't reset; it just pauses
execution.  I know this because the I/O pin doesn't go back to being an
input.  However, the JTAG emulator is unable to "enter a debug state"
which means it can't talk to the JTAG interface on the chip.  Every once
in a while (twice, so far) it connects and I can debug my S/W, but 99.9%
of the time it does not.

This is essentially the same design as a different project I used last
year, but I am having trouble with it now.

Am I breaking some rule here?  Is there anything obvious in what I am
doing that could cause this?

Thanks.