Problems accelerating my van (94 mercury villager) runs great when i first start it up but once engine warms up i have problems accelerating and stalling out when coming from a stop or shifting in to reverse

The engine tickover is too slow when the engine is cold its running on cold start, ie:choke,, after it warms up the engine slows down this is what is causing it to stall in gear,, get some one to hold the brakes hard on and with the engine running in gear ajust the tick over till its reving at 750 - 850 rpm the revs will go up a bit when its out of gear,, before you pick a gear to go into push the foot brake down to stop the car lurching,,,