smoking 97 regal gs /stalling.

I have a 97 regal gs that idles fine for about 10 min or so , then starts smoking from the back of the engine (firewall side) with no room i cant see where its coming from but it looks like its coming from around the exhaust manifold, but no oil leaks under the car. after another 5 mins or so of smoking it starts to rattle,after 2 mins of rattling it finally dies, but wont immediately start until it cools down. it smells like oil. head gasket already replaced. temp never overheats. is something clogged? please help

Welcome to AF.

Check for exhaust system restriction. Possible your CAT converter went South or clogged expecially since you had a head gasket issue.