It sounds like you are adding meat to the pan before it comes fully to temp, which is why your first batch is coming out unbrowned, and you are additionally cooking the steaks to completion in the pan which is why you have too much browning on the second and subsequent batches.
The way to solve this is make sure that your pan is hot for the first batch. I normally throw the pan that i'm going to be searing in on the fire about five to ten minutes before I start to cook but you might need to adjust that time based on type of stove and how quickly it heats. The trick to steaks after that is to sear in the pan and finish in the oven. This lets you control the darkness of the sear independantly of the doneness of the meat. It is critical that you let the pan come back to temp before starting your next piece of meat. Once you have gotten the desired sear on the meat, bring to desired doneness in a 350 degree oven.
Let rest for at least five minutes to allow the steaks to reabsorb the juices.