1999 Chevy Suburban Coolant leak and misfiring.

I have a 99 Chev Suburban with a 5.7 L engine. It has over 200,000 miles on her. Two problems. First one it uses antifreeze. No leaks anywhere on the outside. I put a pressure tester on it and it does leak down but not real fast. I believe it might be internal? Maybe head gasket? The second thing is that it seems to misfire on number three cylinder. I hook it up to a genesis scanner. It shows the number three misfiring but only during idle. I have did the easy checks. Replaced the plugs, wires cap and rotor. I still get the misfiring. Could it be a injector or maybe the head gasket might be causing the misfire. I have been chasing this problem for about six months. Could I get some input on this it would be most appreciative. Thank you. Fred