I am having some problems with a transmission. I will try and describe what it is doing as best as possible.

Keep in mind I just replaced the transmission with a used one out of a junkyard. It is suppose to have 50,000 miles on it. The transmission runs and shifts great the one problem is that every once in a while when I put it into reverse with the brake pedal depressed it will lug down the engine and it will die. Almost like a standard transmission would do if you pressed in the brake with out also pressing in the clutch, but it is an automatic. The Jeep has been setting for about 7 months so of course I will change the plugs, wires, cracked vacuum hoses, oil, fuel filter, and a few other things, but I am not sure what would be causing the problem with the transmission. It has a new tranny filter and new fluid in it. At first I thought it might be a kinked hose or line but it only does this in reverse. Any help would be greatly appreciated.

Is this a cold start?

It doesn't matter cold or hot it does the same thing.

Doesn't anybody know anything about auto transmissions?