Recently, my Avic F90BT has the following problem:

• I turn it on, and when I switch to AM, it has no reception. If I turn it all the way up, all I hear is static, but some stations I can hear the station, so it is "there"
  
• However, if I leave it on, it will eventually "click" and turn on with perfect reception.
  
• When the problem first started, it would come back to life fairly quickly, but now it sometimes takes up to an hour. I thought it was a connection, and it was coming back when I hit a bump or something, but it has come back and started working when I have been stationary, so I believe it is an internal issue.
  

Has anyone ever had a similar problem, or found a fix? IT works fine with FM.

same antenna lead is used for FM and AM on the unit. Many new cars have a built-in FM antenna signal amplifier in-line, and is only powered when radio is on. It could be something loose or dying in your car wiring/circuitry. You will need to check with dealer, installer, or dive into your car's service manual/schematics.

 

It could be the am section of the internal tuner dying also - this can be ruled out by connecting an aftermarket passive single am/fm antenna (or even a coat hanger!) to the unit antenna jack for isolation testing.