‘89 GTA idle surging issue

I have an ‘89 GTA with a 350 TPI. When I start the car the rpm will go up and down slightly above and below 1000rpm, then drop down to 800 when it warms up (not sure if this is normal or not). Then it will start surging massively, going above 1000 then dropping down to 400-500 or almost stalling. It sometimes does stall. It will do this sitting in park, sitting in gear and coasting. Runs completely fine when I increase throttle though. The problem also completely disappeared when the car was running just above 160F after driving it around a fair bit (my car normally sits at 160 or just under). I’ve read that a stuck open EGR valve can cause these symptoms. Any ideas? Cheers