The temp gauge fluctuating and the heater changing from hot to cold sounds like you have air in the cooling system (which makes sense if it's been purging coolant).
After it overheats, is there any hot or cold spots in the cooling system? For example, is the upper hose hotter than the lower hose? One side of the radiator hotter than the other?
A possibility is that there may be some sort of blockage restricting the flow of coolant, causing it to overheat
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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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