Sounds like the guy you got it from ditched his problem onto you. I've seen ar15 run ok with only 2 gas rings, so I don't think its a gas ring problem. Why did the guy replace the recoil spring, and did it run ok before he did that? Have you tried shooting it with a different type of ammo? If you have access to another known good bolt and carrier you could try swapping them out to help eliminate that as a problem. You might also want to check for leaks around the junctions of the front sight base and the barrel and gas tube areas. What is the length of the barrel? Here's a thread where most of the normal suggestions you'll get are covered.http://www.gunco.net/forums/f262/sho...-advice-48568/
For your bolt catch not working, take off the upper receiver and insert an empty mag. Does the tail on the bolt catch that sticks into the mag well get pushed up by the mag follower? Does the mag catch move freely and not bind? If its just a little gritty try flushing it with cleaner and working it back and forth a bunch of times to work out any gunk. If the previous owner has dry fired the gun without the upper in place it may have bent the mag catch and ruined it.