AC only cools when moving, where do I even start diagnosing this?

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 understand what’s going on before I take it anywhere, because I don’t want to be sold a compressor when it’s something simpler. I ended up on the Jeep Service Musaffah blog while searching for answers and the explanation about condenser fans and airflow matched what I was seeing, but I’d rather hear from people who’ve actually had this diagnosed. Was it the fan, a blockage, or something else entirely? And did anyone find it got worse over time, because right now it’s annoying but bearable.