How can I fix my Pioneer KEH 5300?

I've just installed my Pioneer KEH 5300 Radio/cassette after many years of not using. It works OK for about 20-30 minutes until warmed up, then the volume starts to fade and the sound begins to distort and 'clip'. This affects both channels and it gets steadily worse until there is virtually no sound, even at high volume. If I switch off and leave it to cool, it recovers and works again for a while.

Also, there is a loud 'thump' at the speakers when I switch on, - it didn't used to do this.

The unit previously worked fine before going into storage. The cassette, tuner, controls and display are all fine.

Any idea what is at fault here?