My old Cavalier has a Grundig SC303 that has deveoped an intermittent
fault on VHF.

The VHF signal level drops gradually over a period of a few minutes to
the point where no stations are received. AM stations and the cassette
player all work fine.

It usually happens after I've been driving for about half an hour
suggesting a thermal problem or dry joint somewhere in the FM
front-end or the IF section.

Every time I take the radio out of the car and power it up on the
bench it works perfectly - even if I leave it running for hours on
end.
I've used half a can of freezer spray and a hair dryer to thermally
stress the pcb and the components in an attempt to reproduce the fault
but to no avail.
I've tried running it for hours with the covers both on and off.
I've tried varying the supply voltage.

When I put the radio back in the car it always work perfectly for a
while and then the fault returns. Once in fault mode, it often remains
in this state until I take it out of the car whereupon it magically
fixes itself.
I'm pretty sure it's not the aerial since it still works on AM. I've
even soldered some co-ax directly onto the pcb where the aerial socket
connects and tried plugging in a separate aerial once the fault has
manifested itself but it made no difference.

A friend of mine had a Cavalier with the same radio and he tells me
his had exactly the same fault. This suggests it might be a common
component aging problem.
I wondered if anyone else has experienced similar symptoms?

I have the schematics and I'm sure I can fix it if only I can get the
fault to happen on the bench.
I don't really want to buy a new radio since I don't intend keeping
the car much longer.