I recently put a used motor in my discovery and it runs very strong... The down side is she stumbles after she warms up and most recently if I let it sit and idle it does ok for about 20 minutes (800rpm nice and smooth) and then it stalls and will not restart until it is cold??? any ideas what it could be?

 

Classic indicator for crank shaft position sensor. Can confirm by having a spare spark plug and/or a bottle of water. Crank truck, idle until it stalls out. Remove a plug wire, attach to spare plug, lay on manifold, crank and look for sparks - no sparks is the crank position sensor. Or pour a bottle of water on it to cool off quicker, driver side, rear of engine. When swapping sensor make sure you put every nut and spacer back on in exact order. Very sensitive to mis-alignment.

 

Thanks I will try it out tomorrow