Why would make the motor stall out whan put in gear an run rough ?

Firstly we need to know the motor that we are talking about
next a very common problem for boaties ( besides forgetting to put the drain plugs in) is to forget to open the vent on the fuel tanks

sounds like a tired/dirty iac motor (Idle air control.)

You have a broken part inside the transmission, it will need to be torn down and inspected.

Check for broken breather hose from valve cover and follow it under the intake. You may also have a ruptured diaphragm crankcase ventilation valve (vac leak), bolted under intake manifold.

sounds like auto matic choke problem try a manual choke

Fuel filter,coils.

Sounds like there is a problem with the Torque converter in the transmission or Transmission itself. When you put it in gear it should initially go into neutral but occasionally the transmission will stick part way in gear and partially in neutral putting a load on the motor. It is not enough of a load to get the vehicle moving but it is enough to stall the motor. This could be a simple clutch adjustment in the Automatic clutch to a PCV valve issue to a need to rebuild the whole transmission because of a problem with the control body or transmission pump itself. Sorry for the bad news.

I would like more info on the vehicle, as to the engine size. There are vacuum leaks on this engine, both the 4cyl and 6cyl. The injector lower seals are prone to cause this condition, but to check for this and to do it safely, is have it smoke tested with the proper test equipment. The EGR valves can also stick partially open, causing a rough idle and then a idle fluctuation and a stalling problem. Another poss cause is the idle control motor is failing. This vehicle should be taken in for service at a very reputable repair shop. If not properly diag, will cost hundreds of dollars just to "PUT PARTS ON IT UNTIL IT RUNS GOOD".

IT COULD BE THE POWER STEERING PUMP LOCKING UP AND STALLING THE ENGINE. CHECK TO SEE WHICH ONE OF THE PULLEYS IS NOT MOVING AS FAST AS THE OTHERS ON THE PASSENGER SIDE OF THE MOTOR.(WHILE IT'S RUNNING) IT COULD JUST BE AN IDLER PULLEY. I HOPE THIS HELPS TO FIND THE CAUSE OF THE BELT FAILURE. PLEASE KEEP ME POSTED. THANKS FOR USING FIX-YA AND FOR AN HONEST RATING. GOOD LUCK

Sounds like a bad motor mount, pop the hood and see if the motor shakes or moves rapidly when u put it in gear holding down the brake.