Any ideas for solving this problem?

I am currently in a long-distance, open relationship with a very nice Scotsman whom I met last summer. We agreed to attempt our ldr in October. He is kind, fun to talk with, and says that he loves me and wouldn't give me up for anything in the world - at times, I feel overly responsible for his happiness. He's also very charming, romantic, etc. I like these aspects of our relationship okay, but what I really wish I had is a more reliable partner - my boyfriend, who we'll call "David", has flaked on me not once but many times. He has a habit of disappearing and never contacting me for weeks on end. I'll be worried, calling to see if he's alright, and he'll pop back up again and turn on the charm, saying he was right about to call me! Now, I understand that sometimes, a guy just isn't that interested, but what puzzles me is that he frequently goes on at length about how important this relationship is to him, and how he wouldn't make it without me, or how he feels so complete around me, or how amazing I am. I don't think he wants to break up, and to be honest I don't think I really do either, but this strange behavior worries me. The critic in me says: "Be honest with yourself. He's hiding something. Leave him." The romantic in me says: "How can you be so cynical? Take a chance on love." As you can see, I'm rather confused about what to do. I'm also confused by the polyamorous nature of our relationship; after David went to Spain over the winter holiday. I didn't hear from him for a month and a half, and when I asked him if something was wrong he confessed that he'd met a Liverpool girl there who he wanted to continue to see, and reminded me of the earlier agreement I'd made to have an open relationship. I'm more monogamous, really, but I have a ton of poly friends and feel that I *ought* to go with the flow, so to speak. But I'm confused about whether waiting so long to tell me about it was ethical or not. For the curious, I'm 18 and he's about 7 months younger. Advice?