Check engine light, low RPMs, black smoke

I have a 91 grand prix gtp with 320,000 km on it, it has always had a problem where it stalls from time to time but now its getting worse. I have had ALOT of work done to this car and it worked fine before but now its doing this stupid thing. It only really happens when I start it up and I did one day and it had a coolant leak so I filled up the coolant and drove. Well it does this thing where the check engine light comes on. I can drive, but my speedometer stops working, my rpms will not go past 2000 right away even if I put my foot to the floor, and black smoke starts coming out the exhaust. I got the coolant leak fixed by changing my water pump and my map sensor. But it still does it.I settled my rpms a bit by changing the throttle position sensor too but it still does it. If anyone has any ideas on what this could be please help me out.