Does the condenser operate for 6 hours continuously or does the problem occur while it cycles on and off for 6 hours? It could be several things. A thermal shut down or a high pressure trip caused by the heat of the day are possibilities.

When was the last time the AC was serviced?

Thank you mike. Yes, condenser runs continuously from when I turn it on around 7am or so until 1pm or so. Then it shuts down and the central air stays on, but blows warm air. So I just turn it off. I try throughout the rest of the day, but condenser never kicks back on until next morning. It's been quite a few years since it was last professionally serviced. Always have switched out the filters every spring, did the coil clean spray, all the diy stuff and it's always worked fine since. First time it's ever done this.

I recommend you have a HVAC company you trust come and investigate what is going on. The condenser is now 15 years old so a repair would not be unusual. It could be something as simple as a new capacitor.

May be low on freon. It hits the high/low pressure limits in the afternoon.