Car idles rough, when i step on the gas it bogs out, dies @ stops

No idea without my head under the bonnet sorry

Have you checked at all the fuel pressure or the fuel pump?

My Pontiac Grand Prix is also a 99, it has 216,+ thousand miles on it, it runs great, idles good, I need to replace the oxygen sensor tho as it shuts off on me while driving or sometimes coming to a stop. You might try checking the fuel filter and pump as well.

you did replace computer also, the engine you put in will need the computer for it,also get all connections.

bad fuel pump.- just replace it and your good to go!

it could be either a clogged catalytic converter, or a mass air flow sensor. One way to check the cat is to let the car run for about 10 minutes and look at the underside of the car to see if anything is glowing, or check it after a very short drive. To check the Mass airflow sensor locate it and unplug it while the car is running and see if it clears up.

This is a known issue for your model and for GrandAm's. Best to take it to your dealer.

a stop up fuel filter.or water in gas..a bad fuel pump or relay.idle speed control not working.

provided you don't have a vacuum leak, (check all hoses for tightness and cracks with engine running) it's probably your idle air control valve or sensor.

had the same issue with mine, a full tuneup and a really good cleaning for the throttlebody cleared mine up.