I have a FT 727 and have had the same problem. I first noticed about 2 years ago. The longer you go without using the radio, the longer it will work. My first thought was a bad electrolytic or bad VCO board.
I have the service manual, but was unable to determine anything with my limited experience in repair. I would be glad to send you a copy of the service manual to see if you can come up with anything.