1992 Chevy Lumina V6

I have a 1992 Chevy Lumina 114,000 miles. This is becoming a more frequent problem, and is confusing because when it happens I get no warning lights, ie "check oil" "check engine". There's always plenty of gas in the car. When I'm driving, I'm fine for about 20 minutes or so, highway or stop and go, then say I'm driving 60 mph and I start to slow down, the car will start chugging a bit, and as I stop like at a stoplight, it will rev up and chug, and then die on me. It doesn't happen all the time, and I noticed it happens more when the outside temperature is above 40 degrees. Lately, its been happening sooner that 20 minutes later of driving. It will start right back up, but as soon as I put it in drive, it will die. If I put it in neutral at the stoplight it will idle fine, then when I put it back in drive it will die. Sometimes I have to start it three or four times before I can cross the intersection. The car drives fine otherwise. I don't want to be taken advantage of by a mechanic who realizes I don't have a clue what's going on. Please help!