How do I find codes for diagnosis for a 91 buick skylark?

I am trying to figure out why my car keeps on dying when it gets hot. See it will run great till it warms up then it will just studder like a milkshake. If I put oil in it when it is hot it will run just fine. It will start up in park but when I switch it in drive it dies again? Does anyone know what it could be and how do I fix it without taking it to a shop. I am running low on money and I am trying to do this myself. Thanks TonyHow do I find codes for diagnosis for a 91 buick skylark?Tony, first does it flash an engine light? in 91 and cars around that year count the times the light flashes and that coresponds to a code.How do I find codes for diagnosis for a 91 buick skylark?There could be a few things the coil has a crack in it and when it gets hot the crack opens and fails. Or the catalytic convertor is plugged and when it gets hot it plugs it off completely. You can watch your car in the dark and while its getting hot see if the coil arks and shuts the car off, if not look under the car and see if the catalytic is glowing red if so its plugged.