As usual, check for codes (even if the CEL is off).
I'm thinking either faulty Idle Speed Control valve, possibly vacuum leak. Check the TSRM for testing procedure of the ISC, and listen around the engine bay for vacuum leaks with the engine running. I believe an out of adjustment TPS may also cause this issue, testing and alignment procedures are also in the TSRM.
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1989 7MGTE R154 Targa Top White Package w/Blue Interior
Driftmotion 57trim CT26, Upgraded Intercooler, 2.25" Hard Pipes, 3" Turbo Back Exhaust with Test Pipe, HKS VPC w/GM 3 bar MAP sensor, APEXi S-AFC, Walbro 255lph fuel pump, ProSport EVO Series Boost / Fuel Pressure / AFR gauges, Lotek a Pillar Gauge Pod, HKS Type 0 Turbo Timer, Eibach Lowering Springs, Tokico Illumina II TEMS struts, custom powder coated Motegi Racing TrakLite wheels
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