Alright well I called my mechanic to let him know what's going on, and he said it sounds like a brake caliper problem. Maybe one's getting stuck sometimes and when it gets warmed up it expands and unsticks. Does that sound like it could be it? The problem does get worse when I brake, it feels almost as if I were pumping the brakes even when I'm not. How much would that cost to fix generally, if that's it? I'm bringing it down there tomorrow morning, so I guess I'll see in a couple days...

Thanks for the help.