Thought the first. The good thing about an intermittent problem that shows itself after a component "warms up": These are (often, not always!) easy to trouble shoot with a can of "circuit chiller".
Acquire a can (e.g., see link below), pop the top of the tuner, and spray components (transistors, capacitors, chokes, even ICs). If chilling one of them brings the operation back to normal, that is probably the bad one.
Thought the second. I had (have!) a Sangean HD (hybrid digital) FM tuner that had a somewhat similar symptom. It would work perfectly for a minute or less, then the reception would slowly fade away over the next minute or so.
With the plan of doing the kind of brute force troubleshooting described above, I noticed a couple of multi-wire connectors, including one ribbon cable. I reseated them (all) and, lo and behold, ameliorated the problem. :) My guess is that the circuit's heating up caused the connection on one or more of the wires in one of the cables (see arrow in image below) to loosen up enough to cause the fault. This tuner is still working well without intervening interventions :) and sounds pretty darned good, actually.
Offering these thoughts strictly "as is" and for what it is worth! I have never seen an Amber tuner -- although I did once have a rather nice little Amber preamp, which I passed along to the late, great AA alum "LWR" (Larry) when he was in need of one. :-|
all the best,
mrh
