I had the same issue, especially on hotter days. I figured it was the placement of the air filter. It sits in the hottest area of the engine bay, so the air it has to digest is probably very thin. When driving, the cooler air reaches it, but once stationary...
Anyhow, I decided to make a "cold air" intake. I bought a flexible air hose (similar diameter as the air filter), a carbon air intake/filter thingy and a heat blanket to shield the hose from the exhaust pipes (it routed along the side wall next to the exhaust). The air comes from above the radiator (used a scoop to expand the inlet area). It looks crude-ish, but work perfectly well. Feels like it pulls harder a bit. On a side note, It now guzzles more fuel too. I don't have a picture of the contraption handy unfortunately. ☹️