Water on the ignition components (leads, dizzy etc)!!!
My 3.2 Carrera use to do that exact same thing after a wash/heavy rainfall. Once the water dried off, she'd run perfectly.
To test that theory, I once used a hair drier on the dizzy (after a wash) to dry up the water around the plug leads. Never had the problem after a blow dry.
However, since fitting the new Magnecor leads, the problem hasn't re-surfaced 
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Merv
'89 911 Turbo Cab
Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition
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