The BeoMaster 5500, a temperamental beast indeed. Sounds like a classic case of gremlins lurking within its aging circuitry. Wonder if the phase-locked loop is feeling its age? Perhaps the internal capacitors are drying out, causing instability until they reach operating temperature. I had a similar head-scratcher with an old synthesizer that refused to hold a tune until it had been powered on for a good half-hour, a frustrating experience similar to trying to skillfully maneuver in @Slither io with lag. Turned out to be a flaky voltage regulator.
