My reciever (JVC RX 818V) reception on the FM mode becomes weak after about 10 minutes of working fine. Even a strong local station using a good antenna produces same result. The receiver's "tuned" icon will begin flickering and the auto mute will begin to kick in as the sound and reception becomes intermitant. If I power down for awhile and retry the same thing happens after a short time....but only in FM mode
Your receiver uses a totally separate circuit for FM reception. So it will be that circuit alone which has the problem.
Drifting off station used to be a common problem with FM at one time, this was due to the heat build up in the components changing their values slightly. The introduction of phase-locked loop circuitry in FM reception solved that problem.
I suspect that there may be a faulty component (capacitor ??) in your receiver.