Why does my car stereo only work for about a minute before it powers off by itself?

[IronWill82]

I have a '95 Mitsubishi Montero with the factory radio still in it. It's hooked up to a disc changer in the back as well. It has worked great until we recently had to take it to a shop to get some serious engine work done (replaced the engine). Ever since we got it back, when we turn the radio on and put the security code in, the stereo only works for maybe 30 seconds to a minute then it turns itself off and the antenna goes back down. When we turn it back on, we have to re-enter the security code and again, it will only stay on for about a minute. Also, when it is on for the little time it comes on, the volume knob will not work...the volume remains at a fixed level and won't adjust.

Anyone have any ideas of how to best troubleshoot this problem?