Stunning car! Congrats!
I'd say you need to get your Motronics idle programming looked into. If I were you, I'd contact Todd Knighton of Protomotive. He's a guru when it comes to re-coding the BOSCH Motronics software, especially for aftermarket forced induction applications.
'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