Running hot, part 2

1988 Astro Van. Well maintained. Second engine, good history, almost 100K on the engine, so this is not problem associated with engine changing. Ran hot. Changed fan clutch and water pump. Have had it at the mechanics several times, pressure test, no discernable leaks. It loses fluid, sometimes a half gallon on a 15 mile trip. Runs cool, normal at highway speed, then heats up at red light stops. Cannot see any coolant leak on pavement, ever. I see and touch/smell the A/C condensate, but never coolant. At red lights, when it overheats, you can smell the telltale coolant aroma. There is no water in the oil. This driving us crazy, any ideas? Perry