Trevor,

In addition to the other connectors suggested, check also the regular power feed to the head unit. On my 88, similar problems occurred because the metal end caps on the fuse had oxidized. So: remove the fuse(s) - include any in-line fuses or other connectors in the power feed as well - and carefully clean the *entire* contact surface. Even twirling the fuse in the holder won't necessary completely clean the connections; you should remove them for a complete job.

I had the same problem with my wife's '87 240 about six months after I installed an in-dash cd player. It would shut off while driving or when applying the brakes intermittently and then immediately come back on. Even though the battery terminal were clean, it turned out to be the cables. I installed new volvo cables and the problem immediately went away.

Before changing the battery or upgrading to a bigger one you should do some other things first:
1. Check your battery terminals for oxidation. If they are oxidated, clean them with a little sand paper (by the way, clean them anyway you never know) and grease them with some electrically conductive grease (there's a spray with white lithium grease from STP that works OK with battery terminals)
2. Check your alternator to see if it works OK or have it done in a garage. The battery by itself cannot handle the power loads if the alternator fails to charge correctly.
3. Try using a power capacitor (1 Farad should be enough) for power drains that are supposedly coming from the amp.
--
1991 Volvo 245; 1979 Volvo 244

Have you considered instlling a Volvo voltmeter in your dash? They are available on eBay at reasonable cost, fit in the space below the small clock, and are easy to install.

You may be drawing current at the upper margin of the battery/alternator system capacity.

Good Luck,

Bob

:>)