AC only cools when moving, is this a fan issue or something worse?

My Grand Cherokee's AC has been doing this for about two weeks where it's cold on the motorway but goes warm the second I stop at a light, then picks up again when I pull away. It's not dead, just useless in exactly the conditions I need it most. I wanted to learn more about what causes this before I take it anywhere, because I don't want to be sold a compressor when it's something simpler. Has anyone here had this exact pattern diagnosed? Was it the condenser fan, a blockage, or something else entirely? And did anyone find it got worse over time, because right now it's annoying but I'm tempted to just live with it until it cools down outside.