Bad transmission?

Hi, how is everyone. I have a 1992 olds achieva s with the 2.3 quad 4 and and automatic transmission. The car has just under 100k miles and no warning lights on dash are on. The car will run and drive fine, except when it is driven on the freeway for about 10 minutes. When you go to slowdown it makes a noise that sounds like two gears not meshing, Like it is stuck in overdrive or something. Then it stalls if you stop completely. it will start, then stalls as soon as you put it in any drive gear, including reverse. If you let it sit for 10-30 minutes it will drive fine. If you neutral drop it from about 2500-3000 rpm's into low gear, sometimes it will take off and drive normal.

The car seems to have a leaking heater core also, and maybe a bad head gasket, but will drive absolutely fine if its not driven too far of a distance.

Also the temperature gauge will go up past normal and then go back down a little before normal as your driving normally. The voltage gauge also will drop the rise if you turn an accessory on. It has a brand new alternator. Any advice would be greatly appreciated.

P.S. sorry for such a long message
TNX, David