My 2003 Ram 1500 5.7L has a heat issue.

At idle, very low to no heat on drivers side. Passenger side works fine. Heat on drivers side does heat after driving for a while but heat coming out of vents on that side drops when I pull up to a stop light or when vehicle is at idle. What might be the problem? And will I be able to fix it myself? I live in maine where heat while driving is usually a wanted feature.