1994 Toyota 4Runner Brakes are sticking

I put new calipers, rotors, and pads on the front about a month or so ago. Beginning of this week, the front started to stick. I could smell the brakes getting hot. When I had to stop for a traffic light and then started go drive again, everything started working as it should. The problem has become more frequent as the week went on but the s still intermittent. When they stick, the pedal is hard as a rock. It takes almost no pressure and very little movement to get full brake. When they are not sticking, everything behaves as it should. Any ideas?