[A8] Tranny Limp Home + Code 18242

[Rich, Jack]

My 98 A8 entered limp-home mode once before, but without a code when I later checked it.  It did it again recently, and I checked the codes prior to shutting off the car.  It threw the following tranny code:
18242 - Pressure Control Valve 5 (N233): Open or short to ground P1834 - 35 - 00 - -

In both cases it happened while stuck in traffic for a while, and finally starting to go again.  Engine revved strangely high (like no clutch engagement) and then some rock-hard, abrupt shifts, one of which eventually throws it into limp mode.

Fluid completely changed 12K miles ago.  Fluid still full and clean.

Anyone have experience with this, or know what the pressure control valve is?

Thanks,
Jack Rich