1996 Mazda 626 Engine Stalls

I just bought this vehicle last month and was told that it needed a motor mount which is scheduled to get repaired but after a little while when I would crank it up before work and start to drive it, when I would come to a stop, like at a stop sign or light, I would feel it idle rough and then it would go quite and it would repeat this for as long as I was at the light, and then when I would let up off the break and press the gas the engine would rev up but not go and then it would jerk and then go. It only does this like when I first drive the car if it has been sitting for a while. I can pretty much tell now when it is or is not going to stall. My engine light is also on, which I was told something about o2 sensor or catalytic converter. I have transmission fluid. I tried an exercise that you mentioned about a stalling engine. I put the car in drive and it would roll and when I press on the gas the car would jerk and the same for in reverse, but not in neutral. Help!