i am driving '92 740 and had similar problems with the radio/amp. i found that the amp is getting its juice from one place, the music signals from another but there is a separate wire sending a signal from the radio when to turn on or off. on my car this signal was disappearing from time to time and finally it died. it took quite an investigation and few trips to the junk yard (few different amps are laying in the attic). so in my case this commanding signal disappeared within the radio - i rewired it so now it gets juice from the same place the antenna gets its signal. you must be a bit nuts to do such things, but i hate to waste mother nature's resources. the new amp was from more recent volvo and is much more powerful, plus it drives all four speakers, while the original was coupled only with rear ones only. i can try to find some records for the wiring if you are intersted.