FMX Transmission problem??

I have a 70 Mach 1 with a 351W and FMX transmission. Had the whole car apart this winter and working out the bugs at this point. Engine runs great with car in neutral or park. Car runs great driving down the road in any gear. Problem is coming to a stop in gear. Motor will not stay running. Dies like if you don't put in the clutch on a manual transmission car. When sitting in neutral and putting the car in gear it will sound terrible and vibrate alot then finally die. Its like the transmission doesn't slip enough when coming to a stop. What causes this? Is my transmission on its way out? It doesn't slip at all when shifting. Is there a way to adjust this? Theres a vacuum hose going to the trans. What does this do? Does the device the vacuum hose go possibly be shot? You guys have been a great help in trouble shooting other problems in the past Thank you

Been doing a little research. Do the bands need to be adjusted? It ran alright before I took the car apart. I did not do anything to the trans besides draining it and putting new fluid in it. Ditto for the motor. Your help is appreciated.

Could it be my torque converter? Could it be bad? Not enough fluid getting to it? Car will not idle with it in gear. Any ideas?

Thanks for the input Should I use starter fluid to spray and look for leaks?