Icom IC-471A/E/H Service Manualhttp://www.ko4bb.com/manuals/index.php?dir=ICOM
Assuming the internal DC voltages are stable (well regulated),
You have a component (resistor, capacitor), in the squelch circuit, that is changing value.
This value change could be a thermal (heat) issue OR a leakage issue (capacitor).
Standard troubleshooting with service manual, schematic diagram and
bench test equipment required.