I feel for you, oldgrape. My sister had an almost identical situation with her ex; he never showed up when he was meant to see L (my niece), turned up whenever he felt like it or didn't contact her for months at a time. It went on like this for a year and a half and it wasn't doing L any good, she'd see him a few times in a row and then when she started getting used to him again, he'd disappear off the face of the Earth for months.
About a year ago my sister text him to ask when he was going to bother seeing his daughter again, she hasn't heard from him since. We've heard on the grapevine that he's moved away now with his two sons (from a previous relationship).
My sister used to get really upset, as she, too, wanted to do the right thing by her daughter, but we told her that she could only give him so many chances before L was old enough to realise he was letting her down on a regular basis. It is sad, but his side of the family are vile and personally, I think she's better off without the lot of them (that's just what I think, though).
I would do what Chuck has suggested. Your ex is probably getting off on the fact that he's bugging you, too. Take care, oldgrape. x