Lennox G20 BCC does not keep fan running after heat demand satisfied. This is causing heat build up at the heat exchanger and causing the limit to trip. After the heat exchanger limit resets, the unit will come on again. I have replaced the BCC but the symptom remains. Any suggestions

If you have a air flow restriction-- Dirty return filter,dirty evaporator coil or just dirty cups on the blower wheel. All will keep the system from moving the air it needs to and cause a high limit trip.

I've cleaned it up pretty well. The fan does not run for the preset time after the gas valve closes (currently set for 270 seconds). If I manually run the fan for a minute or two, immediately after the valve closes, the high temp switch doesn't open and the next cycle comes on as expected.
The switch only opens on the residual heat.