A/C compressor on heat pump works fine, but occasionally it runs and the airhandler fan does not cut on, causing outside unit to freeze. If I turn thermo switch off and back on, it usually corrects the problem and it works fine until it fails to cut on again, maybe 5 minutes, or maybe a week. If I set fan switch to "on", I have no problems. Any ideas? I have already replaced the fan capacitor and fan motor (thinking it had a dead spot).

Since you have replaced the fan motor and cap, the problem may be either the fan relay, which does the switching of the line voltage to turn on the fan, or may even be the fan contacts in the thermostat, which controls the fan relay.

With a volt meter, you can see if you have 24 volts at the coil of the fan relay with the stat calling for fan, if you have 24 volts, then the relay is defective, if no 24 volts then the stat is defective, or the wiring from the stat to the relay has a problem.

Hope this helps.

It was great to see a response and I thank you so much.