is my hu bad?

I have an Alpine CDA-9811 installed in an otherwise stock '99 Honda Civic. The HU was in the car when I bought it 6 years ago and until a few weeks ago, I never had any issue with it. The radio is working intermittently. The display lights up but when using AM or FM as the source, there is not always a signal. It will usually work for a period of time, sometimes several minutes and sometimes a few seconds, before I hear a "pop" like someone abruptly turning off an amp mid song, and then there is no audio. If I then select CD as the audio source, I can hear audio with no issues. To hear the AM and FM bands again, I have to turn the HU off and back on one or more times. I read post after post about protect mode and HU's not getting enough power at high volumes, but that doesn't seem to fit my situation. The CD portion always works, but the radio tuner doesn't. Any ideas?

Sounds like a bad antenna connection.  Check the antenna for rust and make sure the connections are all good.  It is possible the tuner in the head unit is gone too.Support the12volt.com

I unplugged the antenna and the radio reception was still good enough to hear until the audio cut out again. The HU is connected to the factory harness with an adapter harness, so there are no splices or crimp connectors, just plugs. I unplugged everything and checked inside the plugs for bent and misaligned pins. I put everything back together, turned the HU on again, and physically moved every wire at the back of each plug but the signal never cut out. I physically shook the HU while it was running and tapped on it but it never cut out. The only time it did cut out was after I put everything back together and also as I was initially removing the radio, which could have just been coincidence. I think it is just fried. It's too bad because it sounds good even through my stock speakers. I was hoping to upgrade those if it turned out the HU was ok. Looks like I'll be shopping for a new HU instead.