When you are getting along, it's because this is the easy way for her to get through the day. You ask less questions, there's no need to get into discussions- its just polite pleasantness- like interacting with a work colleague or roommate that you don't like, but you keep the peace. There is no emotional investment. When she's actually trying to argue with you about her issues with you, that's when things are looking up! It means that she sees a glimmer of hope that you might change. If this happens, DON'T ARGUE! Just validate her feelings and empathize with her (well of COURSE you felt that way when I did that [insert random infraction here] sweetheart- that must have been awful!). This is when you make progress. What you currently think are the good times (pleasant distance) are the exact opposite. She is pulling away. Right now she sees another man as the one that can make her happy- not you. You will make no progress- NONE- as long as that guy is in the picture in any way. You have to get him out of your lives so that you and your wife can fall in love with each other again. The longer you wait- you are sealing your fate. Affairs thrive on secrecy- read any article or book you want- they will say this. You have to bring it out into the open and level the playing field. Once this guy's wife knows what you know, you will have a fighting chance. In the meantime, be the best husband you can be. Do everything you can to be the right choice for her. Don't play games with taking off your wedding ring etc.. She needs to know that you aren't a man that runs from his problems. You are a man that takes action for his family. Put the other man's wife and family on notice about what's going on, and what it's doing to your family. GIVE NO WARNING! Your wife wants this to continue- your actions will force her hand and she will get mad and threaten you not to do it. That's why you don't warn her first. Just respond by being a rock solid and loving husband, one who is saving his family.
This is my opinion. There are others on here that may disagree. I'd ask how their marriage turned out. I have my wife back, and she's "in love" with me! In fact, she tells me shes never been happier. She didnt beleive she'd ever love me again- I knew we could get it back. I floundered with this for two years before following this advice. Don't put yourself through what I allowed myself to be put through. Take positive action.