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Weird CIS behaviour

I've been having a hot-start problem intermittently for a few months.  I 
thought that replacing the check valve in the fuel pump had cured it, but it 
happened again last week.

So I decided on a comprehensive day on the fuel injection.  I'd noticed several 
times that the residual pressure falls too quickly when the engine is turned 
off, so I decided to put the gauge on and start swapping bits.  Swapping out 
the frequency valve cured the slow leakdown, so I was fairly happy.
But something odd happened when I was running the diagnostics.  On two 
occasions, fuel pressure fell like a rock when I turned the ignition off.  All 
the way to zero in five to six seconds.  Turning the diagnostic back on, 
waiting the usual four seconds, and then turning it off gave me the approved 
behaviour - pressure retention above 2.8 bar for over an hour.

Has anyone ever seen 'intermittent leakdown' before?  Any ideas?

 Phil Payne
 UK Audi [ur-]quattro Owners Club