dead simple as to why... the radio has a microprocessor (Chip) with an eeprom to store the memory in. this eprom has temporary memory which is lost when power is removed and can be written to LOADS of times, it also has permanent memory which doesn't loose it's data but can only be written to say 10,000 times. the software for the radio writes into permanent memory every volume adjustment, so after a while it starts to loose the memory of the volume setting and hence blasts it out or very quiet. you can get a new microprocessor reprogrammed with a modified software which is readily available, but specialists charge bout £70.
I've tried sourcing this microprocessor but is seems obsolete.
you'll notice it only does it when the unit is cold normally, this is because as it heats up, the material in the eeprom memory is more easily written to.