Heat loss at idle

What is your question?

Hello all, I am experiencing heat loss only at idle. Blows very warm (normal) on acceleration, but turns cool while at idle (stop light). Temp always sits at half, rad fan almost always on, no overheating. Would not think heater core, as heat is warm, no damp floor, or window fogging. Coolant full (not overfull). Vehicle is a convertible. Thanks all for any info.